MOLLUSC A— PELECYPODA. 361 



Phylum V. MOLLUSCA. 

 Class Pelecypoda. Goldfuss. 



(Larnellibranchiata. Blainville.) 



The Pelecypoda or Lamellibranchiata are marine or fresh-water 

 molluscs, with a bivalve shell. The valves are complementary, 

 and are in the majority of species 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 additions 

 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 upper border is the dorsal and the 

 lower the ventral border. The end farthest away from the ob- 

 server is the anterior end ; that nearest, the posterior end. The 

 valves are designated as the right and left valves, respectively. 

 The articulation of the valves is commonly effected by the inter- 

 locking of teeth which are borne on the hinge or cardinal margin 

 of the valves. They are very various, and not infrequently placed 

 upon a hinge plate. In primitive species, and in some specialized 

 types, the teeth may form a continuous row of nearly uniform pro- 

 tuberances and pits {taxodont dentition) as in Area, Cteitodonta, etc. 

 The specialized type of tooth structure consists of a series (3 or 

 less) of short arid stout cardinal (starting at the beak) and (2 or 

 less) long, slender, lateral (not reaching the beak) teeth placed 

 upon a hinge plate (diogenodont — Crassatellites, etc.) and in the 

 most specialized cases with additional structures, such as a 

 roughened area ( Venus), accessory lamellae, extra cardinals 

 (Mactra), etc. (teleodont type of dentition). Other types of den- 

 tition are : coarse, variable, amorphous teeth (schizodont — Unio) ; 

 special curving and interlocking teeth derived from the shell below 

 the beak (isodont — Spondylus, and partly developed in Pecten) ; 

 strongly curving teeth from under the beak, without hinge plate 

 (cyclodont — Cardium, etc.); variously developed hinge structure 

 from exterior ornamentation (dysodont) and . finally, absence of 

 teeth (anodont — Anodontd). 



