262 



MUSEUM OF ANIMATED NATURE. 



[Malleid.e. 



In genrra] also the septa comincnce singly from 

 the cardinal or hinge wall of the valve, and divide 

 into two when about one-fourth the way towards 

 the opposite or lower wall; the thickness of the un- 

 divided part of the septum being equal to or greater 

 than that of the two divisions or layers info which 

 it splits. 



"We can readily understand why the sepia must 

 necessaiily become united together at the point of 

 insertion of the adductor. The muscle never quits 

 its attachment to the valves; while the lobe of the 

 mantle, except in its circumference, and where it is 

 attached to the adductor muscle, must detach itself 

 fnira the surface of the valve which is about to be 

 partitioned off, when it secretes upon the super- 

 imposed fluid the new septum, or basis of support. 

 It 18 obvious, therefore, from the conditions under 

 which the partitions are successively secreted, that 

 they must adhere not only to the circumference of 

 the valve, but to the preceding and succeeding sep- 

 tum, at the part occupied by the adductor muscle, 

 •nd for an extent corresponding to its circumference. 

 The progressive change in the position of this muscle, 

 by the absorption of the posterior fibres, and the 

 addition of others anteriorly, changes in a cor- 

 responding degree the relative position of these 

 subcenlral confluent parts of the septa, and a 

 beautiful undulated disposition of the whole cham- 

 bered part results. If the adductor muscle were a 

 tube, instead of a solid mass, the central confluent 

 part of the septa would of course be perforated, and 

 a siphon would result, the calcareous walls of which, 

 from the proximity of the chambers, would no doubt 

 be continuous, as in man)' fossil jiolythalvwus shells. 



" A disposition is manifested to form chambers, 

 though in a much less degree, in the smaller flattened 

 or superior valve of the water spondylus. In the 

 specimen here deicribeJ there are three chambers, 

 with narrower intervals and much thicker partitions 

 than in the lower valve. The secreting power of 

 the lower lobe of the mantle in the spondylus is 

 greater than in the upper; and the layers of nacre 

 which are successively deposited on the cardinal 

 margin push forwaixl in a corresponding degree the 

 upper valve, leaving a heel or umbo behind the 

 hinge of the lower valve, which, from the inactivity 

 of the secreting surface of the upper lobe of the man- 

 tle, is not opposed by a corresponding umbo in the 

 upper valve." After further details Professor Owen 

 observes that "the interspaces of these successive 

 layers of the growing spondylus cannot, from the 

 absence of a medium of intercommunication, serve 

 any purpose hydroslatically with reference to loco- 

 motion : it is a singular fact, indeed, that the spon- 

 dylus, in which the chambered structure is constant, 

 and the ostrea and other bivalves, in which it is 

 occasional, are cemented to extraneous bodies by 

 the outer surface of the shell, generally by the con- 

 cave valve, so that the septa must be regarded as 

 mere dermal exuviae, still left adhering to the animal, 

 to which, as a motionless bivalve, they are no incum- 

 brance. It is highly probable that all the chambers 

 are oiiginaly filled with fluid, as more or less is 

 found in the outer ones of the specimens brought to 

 this country." 



The fluid, as analyzed by Dr. Bostock, when poured 

 off from a curdy sediment, consisted almost entirely 

 of a solution of pure muriate of soda, with a brown 

 precipitate, to which its peculiar rank flavour and 

 odour were most probably owing. Tlie sediment 

 appeared to be of organic origin. 



Fig. 2886 represents the section of a very old 

 individual, in which the upper shell was very convex, 

 and much more numerously camerated than the 

 specimen desciibed by Piofessor Owen. We need 

 not add that till death the number of these septa 

 and chambers is perpetually increasing. 



We now turn to the subgenus Plicatula, in which 

 the she^l is iiiauriculate ; the upper border rounded 

 and subp:icated, with unequal umbones and without 

 external lacets ; hinge with two strong teeth in each 

 valve : a fosset between the teeth receives the liga- 

 ment, which is entirely internal. 



In our example of Plicatula, Fig. 2887, a represents 

 the hinge of the upper valve, with the two teeth. 



In its distribution and habits Plicatula does not 

 differ from Spondylus. 



The recent species of Spondylidae are numerous, 

 but the fossil species are very much so, and have a 

 wide geological distribution, from the chalk and 

 oolitic beds to the various tertiary deposits. 



Before leaving I he family Sponilylidie, we may 

 revert to the fossil genus termed Dianchora by Mr. 

 Sowerby, in his ' Mineral Conchology.' 



The difl'erent situations in which this genus has 

 been placed seem to indicate that some doubt as to 

 its aflmities exists. Cuvier, in his ' Kegne Animal,' 

 laces it between Pachytes and Podopsis ; M. de 

 ]lainville, between Plagiostoma and Podopsis ; and 

 M. Hang, between Pecten and Pedum. 



M. Deshayes, in his edition of I.amarck, observes 

 that Pachytes (fossil) had been formed at the ex- 

 pense of Plagiostoma (fossil), and goes on to state 



i 



that, from an examination of the species, he was 

 convinced of the identity of those two genera; and 

 moreover that an investigation of the characters of ; 

 Dianchora proved to hiin that they were identical 

 with those of I'achytes and Podopsis. A still further 

 inquiry terminated in the conciusion that the so- 

 called fossil genera Podopsis, Dianchora, and Pa- 

 chytes were only Spondyli whose internal lamina 

 had been^|»solved, the external or conical lamina 

 being thusTeft naked. 



The characters of the fossil shells attributed to 

 Dianchora may be easily perceived by a reference 

 to one of the so-called species (Dianchoia striata) 

 selected by way of example (Fig. 2888). 



The general characters of Plagiostoma, Sowerb. ; 

 Pachytes, Delr. ; Dianchora, Sowerb. ; and Podopsis, 

 Lam., may be seen in Cuvicr's ' Regne Animal,* 

 vol. iii p. 124-5. 



I Family MALLEID.E vHAMMER-OYSTERS). 



In this family, the shell, generally of a black or 

 I dusky horn colour, is inequivalve, inequilateral, and 

 ; very irregular ; the hinge is without teeth ; the mar- 

 ; ginal ligament ^ublinear, simple, or interrupted by 

 crenulations, tni^uscular impression subcentral ; 

 the shell is generally fixed by a bys.ius proceeding 

 from the animal. The mollusk has the mantle open 

 without tube, and prolonged into irregular lobes. 

 The foot is channelled, and almost always furnished 

 at its base with a byssus. 

 j At the head of this family M. Rang places the 

 I fossil genus Possidonia: he characterizes the shell 

 as being very delicate, nearly membranous, equivalve, 

 but inequilateral, oblique, and rounded. The car- 

 dinal border is straight, but a little produced on each 

 side, so as to be auriculafed. Hinge toothless. No 

 pit for the ligaments : no passage for a byssus. This 

 form occurs in the schists of Dillenberg. The spe- 

 cimen. Fig. 2889, is from the Irish limestone. M. 

 ' Deshayes, in his last edition of Lamarck (1836), does 

 not enumerate Possidonia as one of the Malleidae or 

 Malleacea. 

 ! From this somewhat indeterminate fossil form 

 we turn to the genus Vulsella, of which the species 

 are all peculiar to warm climates ; none of them 

 appear to be furnished with a byssus: they are found 

 !' amidst Alcyonia, sponges, and other Polypifera. The 

 general form of the shell is seen in the specimen of 

 Vulsella lingulata, a native of the Indian Seas. 

 (Fig. 2800.) In structure the shell is somewhat horny 

 and delicate; the hinge is toothless, and off'crs simply 

 on each valve a projecting callosity, in which i.-- a 

 pit for the insertion of the ligament. The muscular 

 impression is subcentrical. At Fig. 2890, a shows the 

 valves closed ; i is an inside view, showing the hinge 

 and muscular impression. 

 : The mollusk is elongated and compressed ; the 

 I mantle bordered with two rows of close-set tubercles. 

 ''• The foot is small and channelled ; the mouth is large, 

 the labial appendages much developed and tri- 

 angular; branchiae long and narrow. 



From this genus we advance to an allied genus, 

 viz. Crenatula, of which the mollusk does not appear 

 to be known ; like the Vulselloe, the Crenatula) in- 

 habit the warmer seas, namely, those of the East 

 Indies, New Holland, and South and Central Ame- 

 rica, where they are found in spon^^es, &c., but are 

 neither fixed by their valves nor moored by a byssus. 

 The shell is foliated, flattened, suberjuivalve, in- 

 equilateral, and irregular; a little gaping behind, 

 but without any aperture for a byssus. The hinge 

 is linear and marginal, and marked with crenula- 

 tions which are callous and hollowed into rounded 

 pits for the reception of the divisions of the liga- 

 ment. Muscular impression subcentral. Fig. 2891 

 represents the Crenatula aviculoides. The genus 

 Perna next claims our notice. 



The species belonging to this genus are all natives 

 of the warmer seas, and particularly those of the 

 East Indies; some, however, occur in the seas of the 

 Antilles, at Cape Verd, and the Azores. They are 

 furnished with a strong byssus, by means of which 

 they are moored to rocks, and the roofs and trunks 

 of mangrove-trees growing at the water's edge ; and 

 they appear to range in depths Irom the surface to 

 eight or ten fathoms : the shell is black or dark horn 

 colour, lamellar, very much flattened, subequivalve, 

 inequilateral, and very irregular ; the hinge isstraight 

 and marginal, having on each side a row of small 

 parallel furrows, which are transverse, and in which 

 the divisions of the ligaments are inserted ; the 

 muscular impression is subcentral. 



The animal is compressed; the foot is small, with 

 a byssus at its base. 



Fig. 2892 represents the Perna Isognomum, a 

 native of the Indian seas : a, the valves closed, 

 showing the byssus ; b, an inside view of a valve, 

 showing the hinge and muscular impression : though 

 in the crenulations or pits of the tiinge the genus 

 Perna approaches Crenatula, yet in general form 

 and in presence of the byssus it more closely ap- 

 proximates to fl.c typical genus of the family, viz. 

 Malleus, or the Hammer-headed Oysters. j 



In the genus Malleus the shell is foliated, black or 

 corneous, and of a somew hat nacreous texture ; the 

 valves are subequal, the term irregular, often auri- 

 culated, and presenting a hammer-shai)e or that of 

 the letter T. The umbones approximate, and before 

 them is a notch for the passage of a byssus. The 

 hinge is linear, very long and toothless, with a co- 

 nical oblique jiit, partially external for the reception 

 o' the ligament, which is triangular and subexternal. 

 The muscular impression is of considerable size and 

 nearly central. 



The mollusk is compressed, with a fringed mantle 

 prolonged backwards ; the foot is distinct and chan- 

 nelled, with a byssus springing from its base. 



The Mallei aie natives of the seas of the East and 

 West Indies, and of Australa-sia. They are found gene- 

 rally at or near the surface, to six or seven fathoms; 

 but according to M. Hang, the species from Gua- 

 daloupe and .Martinique occur at great depths. They 

 are moored by the byssus to submarine rocks and 

 other bodies. The species are not numerous, but 

 as two specimens seldom agree in shape, the genus 

 at a cuisoi-y glance might seem to be very exten- 

 sive. M. Deshayes states that he never saw twc 

 individuals of the same species alike. Age eft'ects 

 gieat difterence in their shape, and especially in that 

 of the auricles of the valves. From this circumstance 

 some yet regarded as species may prove to be only 

 the young of others. 



The example selected is the Common Hammer- 

 headed Oyster (Malleus vulgaris}, from the Indian 

 and South Seas. Fig. 2893 : a exhibits the shell 

 with the valves closed ; b, the inside view of a valve, 

 showing the hinge and muscular impression. 



We now pass to a singular fossil form, referred 

 to this family, viz. the genus Inoceramus, Sowerby, 

 in which the shell is characterized as gryphoid, that is, 

 with incurved umbones, inequivalve, irregular, sub- 

 equilateral, with a lamellar shell pointed anteriorly, 

 and enlarged at its base. The umbones are in- 

 curved towards the hinge, which is short, straight, 

 and narrow, with a series of crenulations gradually 

 decreasing for the reception of a multiple ligament. 

 Several species of this Ibrm occur in the chalk, two 

 in the chalk marl, two in the gault, one in the lower 

 green-sand (Slianklin), &c. 



The species Inoceramus sulcafus. Fig. 2894, 

 selected as our example, is from the blue marl of 

 Folkestone. The larger figure is of the natural size ; 

 the smaller shows the hinge of one valve, the other 

 valve being an inside cast. 



Another genus known only in a fossil stale is <' 

 Catillus, of which the specimen selected (Calillus 

 Cuvieri) at Fig. 2895 is a good example. With 

 othei-s of the genus it occurs in the white chalk of 

 England and France. Some of the Catilli in past 

 ages attained an enormous size, specimens having 

 been found measuring several feet in length. 



The hinge of the shell in this fossil genus has its 

 border furnished with a row of small cavities, re- 

 presented at a. 



Among the fossil form of the Malleidas may be 

 enumerated Pulvinites. The shell is delicate, sub- 

 equilateral, with the umbones inclined a little for- 

 wards. The hinge presents eight or ten divergent 

 teeth, forming so many pits. The impressions of this 

 form occur in the chalk. We select the Pulvinites 

 Adansonii, Defr., Fig. 2896, as an example. 



To the Malleida; most conchologists refer the 

 genus Gervillia, like the preceding, found only in a 

 fossil state. This lossil I'oim was first characterized 

 by M. Defrance, from a species in the baculite kme- 

 stone of Normandy, under the above title, from M. 

 de Gerville, who originally detected it. 



It would be out of place were we to follow out 

 in detail the opinions ol different naturalists respect- 

 ing the affinities of Gervihia, but referring to the 

 first vol. of the ' Zool. Journal,' in which these 

 opinions are discussed, we observe that the writer 

 of the article is disposed to regard the Gerviliiic as 

 having the nearest affinity to the Perna;, from w hich 

 they may at once be distinguished by possessing an 

 apparently inner additional hinge, formed of several 

 oblique teeth, variously disposed according to the 

 species. 



Mr. Sowerby considers Gervillia to be interme- 

 diate between Perna and Avicula ; Cuvier gives it 

 (' Regne Animal') as a subgenus or section of Perna. 



The shell is oblong, nearly equivalve ; very inequi- 

 lateral, and oblique. The hinge is long and linear, 

 nearly straight, with many irregular and somewhat 

 transverse little jiits, and teeth placed below the 

 dorsal edge. A better idea, however, will be con- 

 ceived of the general form of the shell by referring 

 to our selected examples : Fig. 2897 represents Ger- 

 villia aviculoides ; Fig. 2898 represents the Gervillia 

 solenoides. 



This genus occurs through various strata upwards 

 from the lias to the baculite limestone of Normandy, 

 and consequently existed through several geological 

 periods ; specimens are found in the green-sand or 

 cretaceous group, and in the oolite. 

 The families or groups which we have hitherto 



