926 THE PALEONTOLOGY OF MINNESOTA. 



[Oarlnaropsls. 



Genus CARINAROPSIS, Hall. 



Carinaropsis, HALL, 1847, Pal. New York. vol. i, p. 183. 



Phmgmostoma, HALL, 1861, Fourteenth Kept. N. Y. St. Cab. Nat. Hist., p. 94; not Waagen, 1880, 

 Pal. Indica, ser. 13, pt. 2, p. 131. 



For generic characters see page 857. 



This is a well-marked genus, and one that we find it difficult to place satisfac- 

 torily in our scheme of classification. Taking the small inner volution alone, we 

 are reminded sometimes of Cyrtolites, at other times of Oxydicus, but the whole shell 

 with its greatly expanded and peculiarly constructed aperture, is so widely different 

 from these genera that, with our present knowledge, we cannot think even for a 

 moment of seriously comparing them. There are perhaps better reasons for 

 bringing the genus into connection with Bellerophon, there being in reality scarcely 

 a single if indeed any feature of Carinaropsis that is not also present in some 

 form or other in species of that geuus. Thus, picking out the more essential 

 characters of Carinaropsis, an expanded aperture is frequently present in Bellerophon, 

 though the inner volution or volutions are never minute as in Carinaropsis, the 

 inner lip is often thickened within into a blunt ridge (e. g. B. troosti) but the ridge 

 is never developed into a projecting wide plate; then the volutions are often 

 carinated, though but rarely, if ever, so distinctly as the smaller volution of Carin- 

 aropsis. Finally, the shells of both genera have a slit-band and a sinus in the outer 

 lip. Though their features on the whole are the same in kind, they still differ 

 so greatly in development that, especially when resemblances in very different 

 directions are considered, we are more than satisfied that Carinaropsis represents a 

 distinct family. 



We have already alluded (see page 857) to certain striking agreements in 

 structure existing between Carinaropsis and Pterotheca, a genus of Silurian shells 

 that all authorities now place with the Pteropoda. In both of these genera the 

 aperture is broadly expanded, a septum is developed, and the back is carinated, in 

 each case more strongly in Pterotheca than in Carinaropsis. But while in the latter 

 the shell forms as much as two volutions, in the former it is merely arcuate, the 

 curvature in no case amounting to a single volution. Though the differences 

 between the two genera are doubtless important, are we not so far justified in 

 claiming that Pterotheca is not farther removed from Carinaropsis than this genus is 

 from Bellerophon? We say "so far" because Pterotheca possesses one character that 

 is not represented in Carinaropsis nor in any true member of the Bellerophontacea. 

 Namely the apical extremity of Pterotheca is divided by two small vertical septa into 

 three portions of which the central one is longer and somewhat wider than the 

 lateral ones. 



