424 CEETACEOUS PELECYPODA 



The shells arc ovate or rounded, more or less equilateral, equivalve or in- 

 equivalve, with cue valve attached, or with both free, the valves being auricled at 

 both sides of the beak, and the anterior ear of right valve generally provided with 

 a short byssal sinus at its base; ligament marginal, cartilage internal, usually 

 filling out a triangular pit in the umbonal region ; adductor scar rounded. 



The number of fossil species belonging to the present family is very great ; 

 they already appear in the oldest sedimentary deposits and continue up to the pre- 

 sent time, there being over 200 recent species known. Comparing with this the 

 numerous fossil species in the different formations, it is seen that those of the 

 cretaceous period are more than equal to it ; during the eocene formation it is con- 

 spicuously less. On the whole, the fossil species are much more numerous than the 

 recent ones. 



1. Avic2ilopecten, M°Coy, 1855, (British Palseoz. rocks and fossils, p. 392). 

 Shell somewhat inequivalve, more or less inequilateral, being often extended poste- 

 riorly, with both valves moderately convex and broadly rounded ; anterior ear flat- 

 tened, smaller than the posterior, and with a byssal sinus at the base ; posterior ear 

 rarely projecting beyond the margin of the shell, often with a shallow emargination ; ■ 

 ligament placed in a narrow foss along the hinge margin, and not in a median 

 cartilage pit as in Pecten. Type, A. concavus, M^Coy, from upper palaeozoic rocks. 



Most of the species of this genus are of a broadly rounded form, and the sur- 

 face is covered with very numerous radiating ribs, as in Fecten ; but the ligamental 

 fosset appears to resemble that of many Aciculce and Fterinece. Meek (Americ. 

 Journ., Sc. and Arts, XXXVII, 1864, p. 217), says that the structm'e of the shell of 

 Aviciilopecten is tabular, as in iheFECTiNiD^, and not prismatic, as in the A ticulidm; 

 but it may be doubted whether that is suflBcient ground for excluding the present 

 genus from the family A viculid^. 



All the species of Aviculopecteii as yet known are from palaeozoic beds. 



2. Fernopeclen, Winchell, 1865, {vide Woodward's Man., Appendix, p. 65). 

 Subequivalve, inequilateral, with subequal ears, surface almost smooth, hinge-line 

 straight, with a central cartilage pit and a number of smaller pits on either side, 

 diminishing in size and depth from the centre outwards. Type, P. Umceformis, "W., 

 from carboniferous rocks. The species of this genus are externally allied to 

 Amusium, which also has sometimes a few oblique ridges on the internal side of 

 the hinge-line, but not such a regular series of pits and tubercles as described in 

 Fernopecten. It is, however, doubtful that those pits are intended for the recep- 

 tion of portions of the ligament, they are probably simple interlocking teeth, but 

 if they really were ligamental pits, the genus ought to be placed in the melininm. 



3. Fecten,* Klein, 1753. Shell suborbicular, or somewhat higher than long, 

 equivalve, or very nearly so, closed ; surface generally with radiating strite or ribs ; 



* Klein was the first author who used the word Pecfe» in a strictly generic sense, though he did not dis-. 

 criminate very accurately between the species ; but his classification of the shells allied to Pecten, as typified by 

 P. varins, Linn^, is admirable. He figures as tjiucal species a form which is very closely allied to, or identical with, 

 P. miles, Reeve. 



