ACEPHALA TESTACEA. 373 



adhere to all sorts of bodies, [and their form is generally modified by the surface of the objects on which 



they grow]. 



M. de Lamarck separates from the Spondylus his Plimlula, from having no external area, or disk, between the 

 beaks ; and flat, almost equal, irregular, plaited and scaly valves, as in many Oysters. [Bp. plicattu, Grael., is the 

 type.] 



Malleus, Lam. — 

 lias a simple fossa for the ligament, as in Ostrea, with which genus Linnaeus left this one, and the more 

 so as the shell is also ineuuivalve and irregular, but it is distinguished by an emargination on the side 

 of the ligament for the passage of a byssus. 



The best known species (Ostrea malleus, Linn.), a rare and dear shell, has the two sides of the hinge extended 



so as to form n ithing like the head of a hammer, while the valves, elongated in a transverse direction, repre^ ol 



the handle. It inhabits the Archipelago of India. Other species, which are, perhaps, but the young of the Malleus, 

 have no hammer-head, and these we must he careful not to confound with the Vulsella;. 



Vulsella, Lam. — 

 Has in the binge, on each side, a little lamina projecting inwards, and it is from one of these laminae 

 that the ligament, similar in other respects to that of the Oyster, is stretched to the other. On the 

 side of the lamina is a sinus for the egress of the byssus. The shell is elongated in a direction perpen- 

 dicular to the binge. The species best known inhabits the Indian Ocean. 



Perna, Brug. — 

 Has across the hinge several parallel fossae opposed to each other in the two valves, and lodging as many 

 clastic ligaments : their shell is irregular and foliated, like the Oysters, and has on the anterior side, 

 underneath the hinge, an emargination, through which the byssus passes. Linnaeus left thetu also 

 among his Ostreae. [The recent species are brought from the Indian Ocean, and from New Holland.] 



There has been recently separated from Perna, the Crenatulte, Lam., which, instead of transverse fossa on a 

 broad hinge, have little oval ones quite on the margin, where they occupy little breadth. It does not appear that 

 there is any byssus. We lind them often buried in sponges. To the Perna?, it is supposed, we must approximate 

 some fossils which have more or less numerous fossa? in the hinge answering to one another, and appearing also 

 to have given attatchment to ligaments. Thus the Gervill'ue, Defr., have a shell almost similar to Vulsella, but 

 with a binge in some degree double; the exterior with opposed fossae receiving as many ligaments, and the interior 

 garnished w ith very oblique teeth on each valve. We find the casts of them w ith Ammonites in compact limestone. 

 [Many species have occurred at various geological periods from the lias upward, to the baculite limestone of Nor- 

 mandy.] The Inocevamut, Sower., is remarkable for the elevation and inequality of the valves, of which the 

 summit is hooked near the hinge, and whose texture is lamellated. The Catilles, Brongn., have, independently of 

 fossa?, for the ligament, a conical furrow drawn in a varix, which is bent at a right angle to form one of the margins 

 of the shell. The valves are nearly equal, and of a fibrous texture. They appear to have had a byssus. The I'nl- 

 viuitet, Defr., have a triangular regular shell, and its fossae, few in number, diverge within from the summit. 

 Their casts are found in chalk. 



The second subdivision of the Ostracea, as well as almost all the bivalves which follow, besides the 



single transverse [or adductor] muscle of the preceding genera, have another muscle going from one 



valve to the other, and placed in front of the mouth. It is apparently in this subdivision that we must 



place 



[The Mulleria, De Per., — 



1 » le of the most singular and rare of known genera. It is remarkable as being intermediate in its 



structure between jEtheria and Ostrea, and as apparently connecting the regular freshwater bivalves 



with the irregular marine bivalves (Ostress), and with the genus Kthcna, inasmuch as in the sinus at 

 the posterior extremity of the ligament it resembles the Naiades and the .I".theri;c ; and in its single 

 muscular impression, as well as its general form, it approaches to Ostrea.] 



Etheri.c, Lam. — 

 Ue large inequivalved shells, as, or even more, irregular than the Oysters, without teeth to the 1 1 i 1 1 ir • ■ . 

 and where the ligament, in part external, exists also interiorly. They differ from the Ostrete in bai 



muscular impressions. It is not ascertained thai their animal produces a byssus. Thej have lately 

 b sen discovered in the Upper Nile. 



Avicula, Drug. — 

 l! is a shell with equal valves, and a rectilinear hinge, often extended into wings on each tide, furnished 

 with a narrow mt, and sometimes with small denticulations on that side which i> m \t 



