Report of the State Geologist. 



475 



(Bhaohyprion) have the delthyrium either open or partially 

 dosed byaconvexdeltidium,andtheorenulationsof the binge-line 



restricted to the immediate vicinity of the delthyrium. Gradual 

 progress in these features is observable throughout the late Silurian 



Fig. 35.— The cardinal area of Stropheodonta FlO. 86.— The cardinal area of Stropheodonta 

 (Brachypriori) Leda; showing the few creuu- (Brachyprion) profunda, with more nunier- 

 lations on the cardinal margin near the ous cardinal crenulations. Niagara group, 

 delthyrium. Clinton group. (Billings.) 



and early Devonian species; and in the middle or later Devonian 

 when the genus disappears, the species have the delthyrium' 

 completely closed, the outlines of the deltidium obliterated and 



Fig. 37.— The hinge area of Strophonella Fig. 38.— The cardinal area of Stropheodou ta 



ampla; showing the extension of the cardi- demissa, with the crenulations extending to 



nal denticulations for about one-half the the extremities of the hinge, 

 length of the hinge-margin. 



the crenulation of the hinge extending to the cardinal extremi- 

 ties. There is corresponding progress in the development of the 

 internal features. "With each phase of Stropheodonta there is, in 



Fig. 39.— The cardinal areas of Stropheodonta demissa ; showing^the closed delthyrium. 



the same fauna, a corresponding phase of the reversed shell 

 Strophonella. In a certain sense the genera Kafinesqujna and 

 Strophomena bear a similar relation to each other, both appear- 

 ing and disappearing in geological history at about the same 

 time. 



27 



