ACEPHALA TESTACEA. 371 



together; but there are several which ahvays gape, even when brought as nigh together as 

 possible, either at one or at both ends. 



THE FIRST FAMILY OF THE ACEPHALA TESTACEA,— 

 The Oysters, — 

 Have the mantle open, with neither tubes nor particular apertures. They have no foot, or only a very 

 small one, and are for the most part fixed either by [cementation of] their shell, or by their byssus, to 

 rocks and to other submarine bodies. Those which are free can move only by squirting out the water 

 by a sudden closure of the valves. 



Tlieir first section has but one muscular mass passing from one valve to the other, as we see by the 

 single impression left upon the shell. 



It is supposed that we ought to arrange here certain fossil shells, whose valves do not seem to have 

 been connected by a bgament*, but to have covered each other hkc a vase and its lid, and to have been 

 held together by the muscles only. They form the genus Acardium, Brug., or Osiracite, La Perouse, 

 of which De Lamarck makes the family Rudistes. The shells of it are thick, and of a solid or porous 

 texture. We now distinguish in it the Radiolites, Lara., whose valves are striated from the centre to 

 the circumference. One of them is flat, and the other thick, nearly conical, and fixed. The Spheru- 

 lites, Laractheriet, with the valves roughened with foliations that rise up unequally. And it is guessed we 

 may place here the Calceoltfi, of which one valve is conical, but free, and the other flat, or even some- 

 what concave, so that they call to recollection the figure of a shoe : and the Hippurites, with one valve 

 conical or cylindrical, that has on its inside two obtuse longitudinal crests : its base appears even tn 

 have been divided into several chambers by transverse partitions ; the other valve forms, as it were, a 

 lid. The BatoUfhes, Montf., are cylindrical and straight Hippurites ; they are often very long ; but 

 there remains much uncertainty on the nature of all these fossils. 



As to the Testaceous Acephales, known in a living state, Linnteus had united under the geniis 



OsTREA (the Oysters) — 

 All those which bad neither teeth nor transverse laniinsin the hinge, the valves being held together by 

 a ligament lodged in a little cavity on both sides. 



The Ostrca, Bru<,^., has the li<^ament as just described, and tlieir shells are irregular, inequivalved and foliated. 

 Tliey are affixed to rocks, to stakes, and even to one another, by the most convex of the valves. The animal 

 {Pt'loris, Poll} is one of the simplest of bivalves ; we observe on it nothing remarkable but a double series of ciliffi 

 round the raarg;in of the ctoak, which has the lobes united only above the head near the hinp;e : there is no appear- 

 ance of a foot. Every one is familiar with the common Oyster (0. cdulls, Linn.), which is fished and reared in arti- 

 ficial beds. Its fecundity is as astonishing^ as its taste is agreeable. [Poli says that the ovaries of a single oyster 

 contain 1,200,000 ova.] Amon^ the species of neifj^hbouring countries we may notice the Os. cristata of the IVIedi- 

 terranean ; among those of distant lands, the 0^. parasitica, which fixes itself upon the roots of the mangroves 

 and other trees that grow within the reach of the salt water ; and the Os. folium, which is attached by the denticu- 

 lations on the back of its convex valve, to the branches of the Gorgonia and other lithophytes. 



j\I. de Lamarck separates, under the name of Gii/ph^a, certain Oysters, principally fossil, the apex of whose 

 most convex valve projects much, and is either hooked or in some degree spiral. The other valve is often concave. 

 The greater number of the species appear to have been free, but some of them hav(?been seemingly attached by 

 their hooked apices. We know only one recent species {Gtiph. iricarinata). [Sowerby reunites Gryphasa to 

 Ostrea.] 



The Claras (Pec/ett, Brug.) have been properly removed from the Oysters, although they have a similar hinge. 

 They are easily distinguished by their inequivalve semicircular shell being almost always regularly marked with 

 ribs, which radiate from the summit of each valve to the circumference, and furnished with two angular productions 

 called ears, that widen the sides of the hinge. The animal (Argus, Poli) has a small oval foot supported on a 

 cylindrical peduncle, in front of an abdomen in form of a sac hanging between the branchia:. In some species, 

 known by the strong sinus under their anterior ear, there is a byssus. The others are not adherent, and can even 

 swim with considerable velocity, by flapping their valves together. The cloak is surrounded with two rows of fila- 

 ments, several of those of the exterior row being terminated by a little greenish globule [with a metallic lustre]. 

 The mouth is garnished with many branched tentacula instead of the four usual labial laraiuje. The shell of the 

 clams is often coloured in a lively manner, [and many species are remarkable for the difference in colouring 



* TM. Desmouline Ims endeavoured to prove that ttiese shells form + S:rhfrulitfi now embraces the Rndir)tUef and BiTt/striWi of Lam., 



a class intermediate bftueen the shellcss Acephales and the Cirrho- with J.,ii^,^nij of Defrance.— Rn. 



podes. Deshayes, on the contrary, asserts that ttiey arc true Bivalves, t [Sowerby and Raa^r maintain that Calceola is much more nearly 



,.illied to Chama, BlainviUe and Rang collect them into a distinct or- allied to Terebralula.] 

 der of Bivalves, under the name of Rndistes.] 



