274 



MUSEUM OF ANIMATED NATURE. 



[Ram's-hokn Dickuas. 



In Chama and Diceras, on the contrary, there 

 are two adductor muscles, and the shells are ad- 

 herent to rocks and stones, &c., generally by the 

 flat valve. Still, it may he observed, that thouijh 

 the genus Tiidacna (with the subgenus Hippopus, 

 •which, however, will not stand) is Monomyarian, it 

 has no immediate affinity with the Monomyarian 

 oysters, or SponJyIi. The ligament o( the hinge is 

 external and marL'inal, and the hinge presents two 

 imequal teeth; the molhisk in slmeture more 

 nearly approaches that of Chama and of the Car- 

 diidat than any other. Accordingly M. Deshaycs 

 remarks that it may be advisable to follow the 

 arrangHraent of Cuvier, who ])liices the Tridacnte 

 in the neighbourhood of the ChamsB, as indeed 

 had been previously done by Linnajus; for the 

 Swedish naturalist places under his genus Chama 

 both the ChamiE properly so called and the Tri- 

 dacna;. 



With respect to Hippopus, M. Deshayes agrees 

 with M. de Blainville, who is of opinioi. that this 

 genus must be suppressed, the principal character 

 of which, namely the closed lunule, is valueless, 

 inasmuch as it does not coincide with the structure 

 of the animal, which is similar to that of the animal 

 of Tri<lacna. 



M. Deshayes remarks that Lamarck would cer- 

 tainly not have sepaiated Hippopus from Tridacna, 

 if he had known that some Tridacna; as they 

 advance in age have the lunule much narrower 

 than it is in youth, and had been acquainted with 

 the animal oV Hippopus, which is fijrured by M. 

 Quoy in the ' Voyage of the .Astrolabe ' (pi. 80), and 

 which differs in nothing from that of Tridacna, 

 excepting that the foot is rather smaller and with- 

 out a byssus. 



M. Lamarck thus describes the characters of 

 the shell of Tridacna :— .Shell regular, equivalve, 

 inequilateral, transvei-se, with a gaping lunule 

 (closed in Hippopus). Hinge with two compressed 

 unequal and internal teeth ; ligament marginal and 

 external. 



M. de Blainville modifies these characters, di- 

 viding the genus into two sections. Section I : — 

 Species whose shell is more elongated and more 

 inequilateral ; the anterior part being longer than 

 the posterior ; the lunule widely open in youth for 

 the p:issage of a byssus. Example, Tridacna Gigas. 

 Section 2 : — Species more equilateral ; tho anterior 

 porti'm ^llorter than the posterior, and forming 

 a vast lunule entirely filled up; umbones curved 

 forwards ; a single post-cardinal tooth on each 

 valve. 



The inollusk is oval and cordiform, having the 

 lobes of the mantle united nearly throughout their 

 circumference, leaving three apertures — one ante- 

 rior, corresponding in Tiidacna gigas with the gape 

 of the lunule, and giving passage to a thick byssi- 

 ferous cylindrical foot ; two posterior, one for the 

 admission of water to the bianchisE, the other < 

 for the passage of the egesta. The mouth is oval, • 

 and furnished with large lips, at the extremity of 

 which are two pairs of pointed labial palps. 



These mollusks are natives principally of the 

 ndian seas, and are found at various depths from 

 the surface to seven fathoms. 



23.5 L— The Giant Tridac.va 



{Tridacna Giijas). Giant Clam ; la Tuilce, or le 

 Bdnitier, of the French. The shell is of huge 

 size, transversely oval, with great imbricato-squa- 

 mous ribs; the scales short, arched, and lying near 

 together ; the interstices of the ribs not striated. 



This noble species, which is a native of the Indian 

 seas, attains frequently to enormous dimensions, 

 and otien weighs from three to four hundred pounds. 

 LinniEus mentions a specimen four hundred and 

 ninety-eight pounds in weight. 



A jMS. in the library of the late Sir .Joseph Banks 

 gives the dimensions of a specimen brought from 

 Sumatra, the weight of which amounted to five 

 hundred and seven pounds. We have seen a shell 

 of large size in the church of St. Sulpice, Paris, the 

 valves of which are used as vessels for containing 

 holy water; it was presented to Francis I., by the 

 republic of Venice. V\'e have also lately seen 

 some very large valves set up as ornaments in a 

 gaixien. 



Of the strength of the cable, or byssus, by which 

 a shell of the weight of four hundred pounds moors 

 itself to the rock or coral reef, we may easily form 

 some idea, and also ot the force with which the 

 animal closes its valves on the least alarm. 



Cuvier, speaking of the byssus, says, " it is very 

 sensibly of a tendinous quality, and is continued 

 uninterruptedly by muscular fibres." Does not this 

 statement seem to favour the ideas of M. de Blain- 

 ville respecting the nature of the byssus, to which 

 we previously referred in our introductory observa- 

 tions on the Conchifera? 



The Tiidacna Gigas, independently of its magni- 

 tude and weight, is a very beautiful shell. Its 



li 



inside is of glossy whiteness, and its general form 

 is very picturesque : hence it is sought for as an 

 ornament for grottoes, for flower-gardens, and as a 

 basin for garden fountains, or for the reception of 

 i rills, or little jets d'eau, which sparkle in its stainless 

 hollow. 



The naturalists of the Astrolabe found this species 

 at Carteret Harbour, New Ireland. The natives 

 brought many very large specimens on board, and 

 ate the mollusks, which were abandoned to them, 

 with the utmost relish, and without being pre- 

 viously cooked. They found the species again at 

 Tongataboo, at the Moluccas, at Timor, and at 

 Wagiou. It appeared to prefer rather shallow 

 water. 



It would seem that there are several species, and 

 some of large size, which have been confounded 

 with Tridacna Gigas: certain small species, more- 

 over, have been regarded as the young. These 

 points have been cleared up by M. Deshayes. 



2952. — The Spotted Tridacna 



{Tridacna Hippopus). Hippopus maculatus, La- 

 marck ; Tridacna niaculata, Quoy ; Chama Hippopu.s, 

 Linna?us. This beautilul shell, often used for orna- 

 mental purposes, is a native of the Indian Ocean. It 

 was found by the naturalists of the Astrolabe at 

 Carteret Harbour, New Ireland ; and also at Vani- 

 koro, where they collected specimens left dry on the 

 reefs. 



It is solid and heavy, irregularly ribbed, and sub- 

 squamous. Externally it is white, spotted with 

 purplish red, especially about the middle of the 

 valves and towards the umbones: the lunule is 

 cordate and oblique, of a yellow tint, with its edge 

 on each valve toothed. The valves internally are 

 of the purest white. 



It appears to be very doubtful whether any fossil 

 species of Tiidacna exists. M. Deshayes, in his 

 Tables, records two fossil and seven recent ; in his 

 last edition of Lamarck he reduces the number of 

 fossil species to one. Mr. G. B. Soweiby, in his 

 ' Genera,' states that Tridacna is only found recent, 

 and ill tropical seas, viz. those of India and Aus- 

 tralia. 



From the Monomyarian Tridacnae we pass to the 

 genus Chama, as restricted by modern naturalists. 



In the genus Chama the shell is irregular, with 

 unequal valves, generally lamellated, and foliated 

 or spilled externally : it is found fixed to rocks and 

 coral reels, like the oyster. The umbones are 

 distant, unequal, curled, or involute. The hinge 

 presents one thick oblique and somewhat notched 

 tooth, inserted into the groove of the opposite 

 valve. There are two adductor muscles, leaving 

 lateral impressions. The ligament is external. 



The animal is somewhat cordiform ; the two lobes 

 of the mantle unite, leaving a narrow passage for 

 the foot, and two short ciliated siphons, as we shall 

 find in Isocardia. The foot is truncate and bent. 

 It is generally by the upper valve that the Chamae 

 are attached, and the stiape of the shell is afl'ected 

 by the pressure and I'onn of the substances in con- 

 tact with it. 



In the first vol. of the ' Trans. Zool. Soc. Lond.' 

 (1834) Mr. Broderip makes some interesting ob- 

 servations on the Chamae, of which he describes 

 ten new species, brought by Mr. Cuming from the 

 intertropical shores of America and the islands of 

 the Pacific Ocean. " The shells," he writes, "are 

 attached by their external surface to submarine 

 bodies, such as coral rocks; and shells have 

 been observed at depths varying from points near 

 the surface to seventeen fathoms. These shells 

 appear to be subject to every change of shape, and 

 often of colour, that the accidents of their position 

 may bring upon them. Their shape is usually 

 determined by the body to which they are fixed ; 

 the development of the foliated laminee which form 

 their general characteristic is affected by their 

 situation ; and their colour most probably by their 

 food and their greater or less exposure to light. 

 The Chama that has lived in deep and placid 

 water will generally be found with its foliations in 

 the highest state of luxuriancy ; while those of an 

 individual that has borne the buffeting of a com- 

 paratively shallow and turbulent sea will be poor 

 and stunted. 



" Lamarck has divided the species into two sec- 

 tions, viz. first, those the umbones of whose shells 

 turn from lelt to right ; and secondly, those whose 

 umbones turn from right to left. M. Sander Rang, 

 in his ' Manuel,' has adopted this division, to which 

 I cannot subscribe, because it will not bear the 

 test of observation. Two remarkable instances are 

 now well known of regular bi-valves of the same 

 species, in which one specimen may be regarded 

 as being the reverse .of the other; viz. Lucina 

 Childreni, and an inequivalve Mytilus in the British 

 Museum ; and to come at once to the case before 

 us, the same species of Chama is sometimes attached 

 by the right, somelimes by the left valve ; or in 



other words, m one individual of the species the 

 umbones will turn from left to right, while in 

 another individual they will turn from right to 

 left."' 



From these observations it may be inferred that 

 the difficulty of determining the species is not in- 

 considerable. The Chamae are almost txclusively 

 confined to the hotter seas ; some however occur 

 in the Mediterranean. 



2953. — The Gryphoid Chama 



{Chama Gryphoides). This is one of the Medi- 

 terranean species: it is not uncommon m col- 

 lections. 



The number of recent species is about thirty. 

 The fossil species are abundant: they occur in the 

 supracretaceous groups, particularly in the sub- 

 Apennine beds, and those of Bordeaux and Dax ; 

 in the cretaceous group, and also in that of the 

 oolite. Mr. G. B. Sowerby states that they are 

 found in the London clay, the calcaire grossier, 

 the chalk, and green-sand ; and M. Deshayes enu- 

 merates twenty species as occurring in the Pliocene, 

 Miocene, and Locene (tertiary) periods, — of these 

 four are living species as well as fossil. 



From Chama we turn to the genus Diceras, if 

 indeed it be distinct from Chama. 



Cuvier says Diceras differs from Chama in nothing 

 essential ; the cardinal tooth is indeed veiy thick, 

 and the umbones (les spirales) of the valves project 

 in such a manner as to bear a resemblance to two 

 horns. It is on these points that Diceras is sepa- 

 rated by Lamarck and others from Chama, while 

 othci-s, as M. Bruguiere and M. Deshayes, merge 

 it into Chama. The latter, in his edition of La- 

 marck (see note), says, " In comparing the young 

 Dicerata with the Chamae, there is no difference 

 worthy of note ; but it must be added that in pro- 

 portion as the shells of the Diceras increase in age, 

 the characters of the hinge become more and more 

 exaggerated, yet without such an alteration as 

 renders them entirely dissimilar to what they were 

 in their youth. It may be said, then, that in reality 

 the Dicerata are only Chamae exaggerated in their 

 volume, their thickness, the prominence of their um- 

 bones, and the size of their cardinal teeth." 



There would not then be any inconvenience in 

 uniting the two genera, forming for each of them a 

 section, which would thus have less value than a 

 genus established for each. 



2954. — The Ram's-horn Diceras 



{Diceras arietina). Diceras arietinum. This spe- 

 cies, and the lew that are known, are all fossil, 

 from the Jura beds (terrains .lurassiques). M. 

 Defrance enumerates five species, but M. Des- 

 hayes, in his last edition of Lamarck, refers only 

 two species (D. arietina and D. sinistra) to the 

 present genus or section ; the former from Mont 

 ^aleve and the neighbourhood of St.-Mihiel, the 

 latter from the upper oolite in the vicinity of the 

 last-named place. 



A very beautiful and interesting form, con- 

 stituting the genus Isocardia, next demands 

 attention. 



Linnaeus considered this form as coming under 

 his genus Chama, and M. de Blainville between 

 Tridacna and Tiigonia. 



Although the bhell is free, never being attached 

 to rocks, we cannot help regarding it as allied to 

 Chama and Diceras, while at the same time it ap- 

 proaches the Cardiidae, or cockle tribe. Bruguiere 

 ranged it with the CarditcD, but, as M. Deshayes 

 observes, Isocardia differs from Cardita both in the 

 structure of the shell and the moliusk. In Cardita 

 the lobes of the mantle are separated throughout 

 their length, and are without siphons. In Isocardia 

 the lobes of the mantle are united posteriorly, and 

 provided with two short siphons or orifices, which 

 may be compared with those of Cardia. In Tri- 

 dacna, as we have already observed, there are 

 two short ciliated siphons, resembling those of 

 Isocardia. 



Cuvier says, " The Isoeardife have the shell free, 

 regular, and convex, with umbones spirally convo- 

 luted, and divergent. The moliusk does not differ 

 from that of Chama, excepting that the foot is 

 larger and oval, and that the anterior opening of 

 the mantle begins to assume its ordinary extent. 

 The Mediterranean presents us with one species, 

 of considerable size, smooth, and rufous, Chama 

 Cor, Linn." 



The generic characters are as follows : — 



The shell is sometimes covered by an epidermis, 

 it is very convex in form, globular, heart-shaped, 

 equivalve, but inequilateral ; the umbones are di- 

 varicated, widely divergent, curved forwards and 

 outwards, and slightly spiral ; the hinge presents 

 two flattened teeth ; ligament external and forked ; 

 muscular impressions distant ; the moliusk has the 

 borders of the mantle fringed with fine tentacular 

 papillie ; posteriorly there are two very short siphons 



