Pecten. This, contrary to Linnzus, is adopted by Brugulere as a genus distinct from Ostrea. Its characters are given : a regular bivalve shell; the valves unequal; the hinge without teeth, most commonly eared, with a triangular pit for the liga ment. The hinge shutting by a black li gament fixed in this triangular pit, and the different forms of the shells, are the circumstances which are supposed to war rant the proposed separation.
Pedum. The shell from which this ge nus has been formed, and of which only one species is known, being that which is figured by Chemnitz and Favanne, is semi transparent, unequal-valved, eared ; the inferior valve gaping; the beaks separat ed: hinge without teeth ; ligament ex ternal, and attached to a long and narrow gutter. The upper valve is striated, anti. granulated longitudinally ; but the under valve is smooth, sharply edged, and hol lowed out in one part.
Perna. An irregular, flattish, bivalve shell: the hinge composed of many linear, parallel teeth, disposed in a straight line, across The hinge is closed by a ligament which is attached between the teeth, and which, by its thickness, prevents the teeth from articulating with each other. Ostrea perna, isogunr, ephippium, pictum, and legumen, of Linnzus, are placed un der this Avicula. This genus Lamarck forms from Mytilus hirundo, Linnzus, which is the only shell in the genus. He describes it : an irregular, loose shell, a little gap ing towards its beaks, and having the valves of unequal size. The hinge cal lous, without teeth ; a little oblong pit, which is marginal and parallel with the edge which supports the ligament.
Malleus. The hammer-oyster of Lin. nieus, with two other species, from this genus, according to Lamarck, who gives for the characters of the genus: an irre. gular loose shell, a little gaping near its beaks, having equal valves : a callous hinge, without teeth, having a conical pit, placed obliquely on the edge of each valve. This genus seems to be hardly sufficiently distinguishable from the last ; nor does the name of the genus appear to be well adapted, since some of the spe cies no ways resemble a hammer in their form Vulsella. This genus is formed from the Mya vulsella of Linnzus. The uncer tainty with which this shell has been viewed, at different periods, shows the uncertainty of its nature, and the proprie ty of not placing it under any other genus.
Linnzus had once considered it as a Pin na, and Brugulere as an oyster. It is a loose shell, longitudinal, and nearly equi. valved, terminating in a very short bent beak ; the hinge callous, depressed, and without teeth, of equal elevation on each valve, forming a rounded conical pit for the ligament.
Ostrea. In this genus is comprehended those shells only which are oysters, ac cording to common acceptation. The characters of the genus are, therefore, an irregular, adherent, inequivalve shell ; the hinge without teeth; an oblong pit, sulcated across, giving attachment to a ligament.
Grvphza. The shell, for the reception of which this genus was established by Lamarck, was improperly considered as a species of Anomia by Linnaeus, and as a species of oyster by Brugulere ; to which latter genus it undoubtedly very nearly approximates. It is a loose unequivalved shell; the inferior valve concave, termi nated by a beak turned upwards, and bent into an involved spire ; the superior valve smaller, like an operculum, the hinge without teeth, but with an oblong and dented pit ; and one muscular im pression in each valve. Lamarck enu merates eight fossil species of this genus.
Plicatula is a genus formed, by La marck, from Spondylus plicatus of Lin ?mos. It is an angttlated shell, with une qual valves and unequal beaks, which are pointed; the edges are in deep plaits; its hinge is formed of two strong teeth on each valve, and an intermediate pit for the reception of the ligament; with one muscular impression on each valve.
Spondylus. In this germs of Linnzus are comprised bivalve, irregularly-tOrmed shells, the hinge of which is formed of two strong hooked teeth, and an interme diate pit for the ligament ; in the interior valve are two thick recurved teeth, with two intermediate rounded cavities to re ceive the teeth of the other valve, and an elongated pit, in which the ligament is placed ; in the superior valve are two ex terior cavities for receiving the teeth of the other valve, and two teeth curving backwards, with a pit fur the ligament.