228 AN INTRODUCTION TO THE STUDY OF FOSSILS 



Very characteristic of the Comanchean of Texas and of the 

 Cretaceous of both the western and eastern states. E. arietina, 

 at times, as in the Del Rio clay of the Texas Washita, occurs in 

 enormous quantities, forming solid beds up to six inches thick 

 (Fig. 97)- 



1. Name one similarity to Ostrea ; one difference between the 

 two forms. 



2. At what time in the life of the individual Exogyra does it 

 become attached to a foreign object ? Reasons. 



3. Sketch interior view of left valve, indicating muscle im- 

 pression and ligament groove. 



4. How is this known to be a pelecypod and not a gastropod ? 



Unio (Fig. 98). Jurassic to present. 



Valves of equal size, with a thick, brown, horny epidermis 

 developed as a protection against the carbonic and humic acids 

 in the fresh water as well as against the movement of the water 

 itself. Shell thick, with a hinge line of heavy, indefinitely shaped 

 teeth. External ligament present. There are no true siphons, 

 simply openings between the mantle edges for incurrent and 

 excurrent water. The animal grows slowly and may live fifteen 

 or twenty years. The name is derived from the Latin unio, a 

 pearl ; the shells are occasionally pearl-bearing. 



These are the " river mussels " found at the bottom of lakes, 

 ponds, or running streams in all parts of the world, more espe- 

 cially in the northern hemisphere. They rest erect with valves 

 slightly gaping, partly buried in the mud and sand ; if disturbed, 

 the foot and mantle edge are drawn into the shell and the valves 

 shut tightly. 



The sexes are distinct. The eggs are fertilized within the body 

 by spermatozoa introduced with the respiratory current ; these 

 fertilized eggs are passed into the cavities of the outer gills, 

 where they are nourished by a secretion from their walls. The 

 larvae are thrown out through the exhalent siphon. The 

 presence of these developing young cause the shell of the female 

 to be more arched than that of the male. 



