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- 

 i dial sculpture. 



T No siphons are 



present. Tenta- 

 cles are present 

 on the edges of 

 the mantle. Of 

 especial interest 

 are the bright, 

 bead-like eyes, ar- 

 ranged around 

 the edge of the 

 mantle. They 

 possess cornea, 

 crystalline lens, 

 and retina. Ex- 

 periment has 

 shown them to be 

 sensitive to 

 changes in light 

 and shade, though 

 incapable of per- 

 ceiving objects. 

 They possibly 

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

 movement in the water. 



Fig. 99. — The right or lower valve of a two and a half 

 year old scallop, Pecten gibbus borealis Say. ( X f .) 

 The arrows show how the scallop moves. When the 

 animal forces the water out between the valves at a it 

 moves in the direction of b, when out at c in the direc- 

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



