240 



frequently makesa reentrant curve — the pattial sinus — indicating that 



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

 indicated in figure 151. 



The principal soft parts of the animal comprise: the mantle, consisting 

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

 with the anteriorly placed mouth, and the anterio- ventral /<wZ ; the gills or 

 brancltia, which consist of complicated lamellae hanging on either side of 

 the abdomen in the mantle cavity: and the siphon* — 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 lie verified by dissecting a clam, mussel, or other bivalve. 

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

 besl 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 Paheontology of New York, to which reference is 

 made in The text. 



Gems AVICCLOPECTEX. McCoy. 



[Ety. : From the Genera Axieula and Pecten.~] 



(1851: Annals and Magazine of Nat. Hist., 2d Ser., Vol. VII.. p. 171.) 



Shell with the valves unequal, the right valve usually 

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

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

 tinned forward into the ear, and backward onto the wing, 

 triangular, well-defined and somewhat flattened portions on 

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

 furrows, parallel to the hinge margin. The hinge line \> 

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

 and the surface is ornamented with rays. 



Aviculopecten princeps. (Conrad.) (Fig. 152.) (Pal. 

 X. V.. Vol. V.. Pt. I., Pis. 1., V.. VI., XXIV., LXXI.) 



Distinguishing Characters. — Large size, obliquely 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 the middle of the posterior 

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

 by fine, sharp growth-lines. 



s 



