230 AN INTRODUCTION TO THE STUDY OF FOSSILS 



5. How do you account for the roughened surface at the 

 umbo of most individuals? 



6. What is the function of the teeth ? 



7. How can you usually tell the shell of a female Unio from 

 that of a male ? 



Pecten (Figs. 99, 100). Pennsylvanian to present. 



Shell nearly equilateral, with well-developed ears. Hinge 



line straight with a central pit for the insertion of the internal 



ligament. Only one adductor muscle present. Surface usually 



marked with ra- 

 dial sculpture. 



No siphons are 

 present. Tenta- 

 cles are present 

 on the edges of 

 the mantle. Of 

 especial interest 

 are the bright, 

 bead-like eyes, arr 

 ranged around 

 the edge of the 

 mantle. They 

 possess cornea, 

 crystalline lens, 

 and retina. Ex- 

 periment has 

 shown them to be 

 sensitive to 



Fig. gg. — The right or lower valve of a two and a half changes in light 



year old scallop, Pecten gibhus borealis Say. ( X f.) ^ v. ^ +V. V. 



The arrows show how the scallop moves. When the aUQ snaQC, tUOUgn 



animal forces the water out between the valves at a it incapable of per- 



moves in the direction of b, when out at c in the direc- . . k ' f 



tion of d, when out at e in the direction of/. CClVing ODJCCtS. 



They possibly 

 sense the approach of an enemy by its shadow or by its 

 movement in the water. 



