S37 



PECTINID^E. 



" The secreting power of the lower lobe of the mantle in the Span- 

 dylut," says Professor Owen in continuation, " is greater than in the 

 upper ; and the layers of nacre which are successively deposited on 

 the cardinal margin push forward 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 mantle, is not opposed by a corresponding umbo in the upper 

 valve. 



" The laminae, which are deposited in a continuous series of super- 

 imposed layers at the hinge of the lower valve, are not continued in a 

 like state of superposition throughout ; they soon separate from each 

 other, and do not again unite, except at the space corresponding to the 

 adductor muscle, and at the circumference of the valve. 



" The interspaces of these successive layers of the growing Spondylia 

 cannot, from the absence of a medium of intercommunication, serve 

 any purpose hydrostatically with reference to locomotion ; it is a sin- 

 gular fact indeed that the Spondylua, in which the chambered structure 

 is constant, and the Oitreo; and other bivalves, in which it is occa- 

 sional, are cemented to extraneous bodies by the outer surface of the 

 shell, generally by the concave valve ; so that the septa must be 

 regarded u mere dermal exuviae still left adhering to the animal, to 

 which, an a motionless bivalve, they are no incumbrance. It is highly 

 probable that all the chambers are originally filled with fluid, as more 

 or less is found in the outer ones of the specimens brought to this 

 country. 



" In the Testaceous Cephalopods a new structure is added, namely, 

 the siphon, whereby the exuvial layers of the old shell and the deserted 

 chambers are converted into a hydrostatic instrument, subservient to 

 the locomotion of the animal. The operation of the siphon and chambers 

 has been ably explained by Dr. Buckland in the Nautilus, where the 

 calcareous inflexible tube protecting the membranous siphon is not 

 continuous. The working of the siphon is however less intelligible in 

 those species in which the outer calcareous tube U continued from 

 chamber to chamber, as in the Spirulir, Orthaceratita, &c. ; and it is 

 with respect to camerated shells of this kind that I would ask how far 

 the reasoning suggested by the chambers in the Water-Spondylus may 

 be applicable in their case, and whether a final intention can be clearly 

 traced beyond the diminution of specific gravity occasioned by a large 

 proportion of the shell being converted into receptacles of gas; if 

 indeed we have sufficient evidence to assume that they do not contain 



denser fluid, like the Spondylus." 



The cut represents a section of a very old individual of this species, 

 in which the upper valve was very convex, and furnished with a great 

 number of septa. 



Section or Watcr-Spondylui cut longitudinally through both valves, which 

 are represented 5 closed. 



The species of Spondylu* have been found attached to rocks, corals, 

 other shells, &c., at depths varying from the surface to 100 fathoms, 

 in the seas of warm and temperate climates, as the West Indies, the 

 Canaries, the Mediterranean, India, Torres Straits, the Pacific, and 

 West America. The finest and most beautiful species are natives of 

 tropical and intertropic.il localities. 



The number of recent species is about 30, and fossil 45. The latter 

 are found in the Inferior Oolite and Neocomian Beds. 



fedum, Lamarck, (sub-genus). Animal oval, oblong, flattened, 

 having the lobes of the mantle open throughout their circumference, 

 thickened on their edges, and furnished on this part with many rows 

 of tentacular cirrhi, and, at regular distances, tubercles with smooth 

 surfaces. A pair of large branchiae descending on each side to the 

 edge of the lower border of the mantle ; abdominal mass small, having 

 anteriorly and high up a small vermiform foot, and at its base a silky 

 byssus of some size ; mouth oval, having on each side a pair of labial 

 triangular palps. (Deshayes.) 



Shell inequivalve, subauriculate, lower valve gaping; umbones 

 unequal and distant. Hinge toothless; ligament partly external, 

 inserted in an elongated canaliform pit, which is hollowed out in the 

 internal wall of the umbones. Lower valve notched near its posterior 

 bae. (Lamarck.) 



P. tfpondyloideum is the only species. The shell is ovato-cuneiform, 

 planulate ; whitish tinged with red or purplish, especially near the 

 umbones ; the upper valve flat and striated longitudinally ; the lower 

 valve largest, with the lateral edges turned up and raised above those 

 of the tipper valve. 



Shell of Ptditm Spontlyloiileum, 



There is a smaller variety, much more round, and generally more 

 delicate. 



It is a native of the East Indian Seas (Isle of France, c.), where it 

 is found half-imbedded in coral-reefs. 



Plicatida. Shell inequivalve, inauriculate, narrowed towards its 

 base ; the upper border rounded, subplicated ; with unequal umbones, 

 and without external facets. Hinge with two strong teeth in each 

 valve. A fosset between the cardinal teeth, receiving the ligament, 

 which is entirely internal. (Lamarck.) 



The species inhabit the seas of warm climates, where they are 

 found adhering to stones, shells, and other submarine bodies, at depths 

 varying from four to eleven fathoms. 



Woodward makes the number of recent species six, and fossil forty. 



Shell of Plicatula. a, hinge of upper valve. 



Ostrea (Oryph(ea (Lam.), fossil principally ; Exogyra (Say), fossil 

 only). The two latter genera are so generally established in fossil 

 catalogues, that it may be convenient to retain their names ; but we 

 agree with Mr. G. B. Sowerby and M. Deshayes, that, physiologically 

 considered, they ought to be abandoned. Mr. Sowerby, in his ' Genera,' 

 remarks that the particular points upon which Lamarck depends for 

 his distinction between the Oysters and Gfryph<tre are, 1st, the apparent 

 regularity of the latter ; 2nd, their being scarcely if at all attached ; 

 and 3rd, the generally large, involute, spiral umbo of the lower valve. 

 To the first Mr. Sowerby answers, that though the Gryphcere are in 

 general apparently more regular than the Ostrea, they cannot be consi- 

 dered as regular shells, and that they are moreover very variable ; 

 secondly, the Gryphace, as well as all other Ostrefe, are attached by the 

 umbo of the larger and concave valve, and this particularly in the 

 young state (in which state ia fact it is impossible to distinguish 

 between one and the other); moreover both become free as they 

 advance in size ; and if the Gryph&(e are then apparently more regular, 

 it is because in their young state they have lived in situations where 

 they could only become attached to small regular objects, while the 

 Ostrecc, having lived in more rugged and irregular situations, and 

 necessarily remaining attached for a longer time, have partaken more 

 of the regularity of their native situation. " Indeed," adds Mr. Sowerby, 

 " there is sufficient evidence that an oyster, when by any chance it 

 becomes attached to a small smooth object, where it is comparatively 

 free, becomes also regular in the same degree : thirdly, we cannot 

 approve of the term spiral as at any time applicable to the umbo of 

 the lower valve of the Gryphme ; when young, it is not involute; and 

 though Lamarck mentions the size of this as one important distin- 

 guishing mark of his genus, he gives the characters of several species 

 in which this part is small. Another circumstance in which the 

 Oryphcea is thought to differ from Oitrea has been dwelt upon by some, 

 an obscure lobo or sulcus observable on the right side, particularly 



