96 LIFE ON THE EARTH. 



slightly from fossils which might be selected from 

 the Oolites. 



Trigonia exhibits more diversity of form and or- 

 nament, but preserves the essential characters ; a 

 thick shell, ribbed or tuberculated with beaks bend- 

 ing toward the posterior side, which is angular, and 

 bears a prominent ligament often preserved in the 

 fossils. Internally the shell is nacreous, with deep 

 muscular impressions; the hinge-teeth are strong, 

 radiating, and transversely striated. If we place in 

 the order of their existence the following species of 

 Trigonia, we shall perceive at once the general affinity 

 and the special diversity which runs through this 

 genus. The recent species is the only one with ridges 

 radiating from the beak over all the surface. 



Recent Trig, pectinata. 



Chalk T. sulcata. 



Cretaceous T. dsedalea. 



)T. clavellata. 

 T. costata. 

 T. striafca. 

 Lias T. literata. 



Pteropoda. These floating mollusks are few in 

 the stratified deposits of every age ; more frequent in 

 the sea. Conularia, Theca, and Pterotheca, belong to 

 the Palaeozoic Strata ; Hyalsea, Cleodora, and Cuvieria, 

 are both recent and tertiary. 



