ACEPIIALA TESTACEA. 371 



together ; but there are several which always 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. 



Their 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 ligament*, but to have covered each other like a vase and its lid, and to have been 

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

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

 texture. We now distinguish in it the Radiolites, Lam., 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, Lametherief, with the valves roughened with foliations that rise up unequally. And it is guessed we 

 may place here the Calceolce%, 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 to 

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

 lid. The Batolithes, 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, Linnaeus had united under the genus 



OsTREA (the Oysters) — 

 AH those which had neither teeth nor transverse laminae in the hinge, the valves being held together hy 

 a ligament lodged in a little cavity on both sides. 



The Ostrea, Brug., lias the lis:ament as just described, and their shells are irregular, inequivalved and foliated. 

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

 (Peloris, Poll) is one of the simplest of bivalves : we observe on it nothing remarkable but a double series of cilife 

 round the margin of the cloak, which has the lobes united only above the head near the hinge : there is no appear- 

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

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

 contain 1,200,000 ova.] Among the species of neighbouring countries we may notice the Os. cristata of the Medi- 

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

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

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



M. de Lamarck separates, under the name of Gryphita, 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. 

 Tlie greater number of the species appear to have been free, but some of them have been seemingly attached by 

 their hooked apices. We know only one recent species {Griph. tricarinaia). [Sowerby reunites Gryphsa to 

 Ostrea] 



The Clams (Pecfen, 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 branchine. 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 laminae. The shell of the 

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



* [M. Desmoulins has endeavoured to yrove that these shells form 

 a class intermediate between the shelless Acephales and the Cirrhn- 

 pndes. Dfshayes, on tlie contrary, asserts that tiiey arc true Bivalves, 

 Hilled to Chama. BUinville and Rang collect them into a distinct or- 

 der of Bivalves, under the name of Rudistes.] 



+ .<•; herulilri now embraces the Rndinlitct and Biroslrites of I,a 

 with Jodamla of Defninee.— En. 



t [Sowerby and Ranjf maintain that Calceola is much more nca 

 allied to Tcrcbralula.] 



B B 2 



