206 



NEW YORK STATE MUSEUM 



rotund; cardinal slopes meeting in obtuse angle; young shell car- 



inate toward the beak in the pedicle valve; 

 brachial valve gently convex; median fold 

 beginning as a depression at the beak, and 

 becoming elevated near the front; two plica- 

 tions in sinus, three on fold; sharp rounded 

 plications; strong frontal sinuosity; very fine 

 concentric striae. 

 Found in great abundance in the lower Clinton limestone at 



Niagara. Also east of Lockport (Hall). 



^ 



Fig. 133 Anoplotheca plicatula 



Class PELECYPODA Goldfuss 

 (Lamellibranchiata Blainville) 



The Pelecypoda, or Lamellibranchiata, are marine or fresh-water 

 mollusks, with a bivalve shell. The valves are complementary, and 

 in the majority of species are of nearly similar outline and size. In 

 each valve may be distinguished 

 an initial point, or beak, around 

 which the concentric lines of 

 growth mark the successive ad- 

 ditions of shelly matter. 



The orientation of, most- shells 

 is effected by holding them with 

 the hinge line uppermost and the 

 beaks pointing away from the 

 observer. Thus placed, the up- 

 per is the dorsal and the lower 

 the ventral border. The end far- 

 thest away from the 

 is the anterior end; that nearest, the posterior end. The valves are 

 designated as the right and left valves respectively. The articula- 

 tion of the valves is commonly effected by the interlocking of 

 teeth which are borne on the hinge or cardinal margin of the valves. 

 They vary greatly, but can usually be divided into the short, stout 

 cardinal teeth, which are situated under or near the beak, and the 

 ridge-like lateral teeth. The opening of the valves is brought about 

 by an elastic ligament stretched across the hinge from valve to 

 valve, behind the beak, which acts, on the principle of the C spring, 

 whenever the tension of the adductor muscles, which close the valves, 

 is relaxed. In many genera, an elastic, compressible cartilage, the 



Fig. 134 Diagrammatic view of left valve of 

 Cytherea; (aa) anterior adductor scars; (pa) 

 posterior adductor scars; (pi) pallial line; (ps) 

 observer P^^^^^^ sinus; (t) teeth; (um) umbo; (1) ligament 



