240 



frequently makes a reentrant curve — the pidUnl si mis — indicating that 

 tlie animal had a retractile siphon. The various parts described are 

 indicated in ligure 151. 



The principal soft parts of the animal comprise: the riKmtle, consisting- 

 of two tieshy folds, one lining each valve, and building it; the abdomen, 

 with the anteriorly placed month, and the anterio- ventral /oo< ; WxefjilU or 

 hrandiw, which consist of complicated lamellte hanging on either side of 

 the abdomen in the mantle cavity; and the siphons — present only in 

 certain forms — posteriorly placed, often capable of great extension, and 

 serving, the one for the entrance of the water and food particles, and the 

 other for the exit of the water and waste products. 



Note. — The numerous text-books and laboratory guides in zoology 

 will furnish more detailed information on the anatomy of the animal, 

 which should be veritied by dissecting a clam, mussel, or other bivalve. 

 Zittel's Text-book of Palttontology (Eastman's translation) contains the 

 l)est comprehensive account of the fossil genera. The generic and 

 specific description, of the forms here noted, may almost all be found in 

 Vol. v., Pt. I., of the Palaeontology of New York, to which reference is 

 made in the text. 



Gkm s AVICrLOPECTEN. Mtrov. 



[Ety. : From the Geuei'a Acioila and Pectcn.^ 

 (iSol: Annals and Magazine of Nat. Hist., 2d Ser., Vol. VII.. p. ITl.) 



Shell with the valves unequal, the right valve usually 

 smaller and flatter than the left valve, and having an an- 

 terior "byssal notch." The hinge line is straight and con- 

 tinued forward into the enr, and backward onto the wing\ 

 triangular, well-defined and somewhat flattened portions on 

 either side of the beak. The cartilage lies in several shallow- 

 furrows, parallel to the hinge margin. The hinge line is 

 shorter than the greatest antero-posterior diameter (length), 

 and the surface is ornamented with ravs. 



AvicuLOPECTEx I'RixcEPS. (Courad.) ( Eig. l-j^.l (Pal, 

 N. Y., Vol. v., Pt. 1., Pis. 1.. v., VL, XXIV., LXXl. I 



Distinguishing Clicirncteis. — Large size, ol)li(juelv broad- 

 ovate outline with axis inclined more than sixty degrees to 

 the hinge line; nearly equal length and height; straight 

 posterior slope from the beak to th(^ middle of the posterioi- 

 end; large ear and wing; regular alternating rays, crossed 

 by fine, sharp growth-lines. 



