PLATE V 



Fig. 1-23 . . . Exogyra arietina Roemer. Del Rio clay. 

 Shoal Creek at Austin, Texas. 



Fig. 1-3 . . young normal specimen shown in 

 three positions. 



Fig. 4 .... young individual showing a strong 

 spiral groove. 



Fig. 5. ... aberrant adult specimen with ex- 

 ceptionally coiled beak. 



Fig. 6. ... aberrant adult specimen with 

 strongly coiled beak. 



Fig. 7. ... very young specimen, natural size. 



Fig. 8 .... the same, strongly amplified, show- 

 ing a smooth surface on the 

 greater part of the shell, ribs 

 beginning to appear at the mar- 

 gin. 



Fig. 9. ... very young specimen, natural size. 



Fig. 10. ... the same, strongly amplified, show- 

 ing the smooth surface of the 

 shell at this stage of develop- 

 ment. 



Fig. 11.. . .very young specimen, natural size. 



Fig. 12, 13 . the same, strongly amplified to 

 show the ornamentation. The 

 shell is smooth on the point of 

 the beak, and develops radial 

 ribs farther down. 



Fig. 14 .... very young specimen, natural size. 



Fig. 15, 16 . the same, strongly amplified to 

 show the ornamentation. The 

 shell is smooth on the point of 

 the beak, and develops radical 

 ribs farther down. 



Fig. 17. ... adult specimen twice amplified to 

 show the ornamentation on the 

 point of the beak, and its simi- 

 larity with that of the small in- 

 dividuals illustrated in fig. 11- 

 16. 



Fig. 18-23 . upper valves of small specimens. 

 The slit apparently limiting the 

 umbo in figure 19 is due to frac- 

 turing and is not natural. 



