THE NAUTILUS. 



23 



and similar shells are before me from the Wisconsin river, Sauk Co., 

 Wia.,and the Mississippi River at Davenport, la.; while in the Kas- 

 kaskia and Spoon rivers, Ills., a shellmore broadly triangular in form 

 is found, which apparently represents the extreme in that direction. 

 The Alabama form is closer to typical undatus, but like many of the 

 northern species, that have obtained a foothold in that system and 

 retained their specific identity, is apparently uniformly smaller and 

 resembles a half-grown example of the common Mississippi expres- 

 sion of the species. 



But taking the series thus specified as a whole, it exhibits a similarity 

 throughout, which is congruous only with specific identity and which 

 separates it as a whole from union with any other described species. 

 And in this assemblage and united by a series of unmistakable in- 

 termediates are the undatus of Barnes and the trigonus of Lea. 



If, in addition to what has already been said, anything further is 

 needed to prove the identity of the two forms, the following com- 

 parison of the specific characters of the two species as given in the 

 original descriptions will show that there is no ground for question- 

 ing their specific identity. 



Undatus. 



Subtriangular. 



Disks swelled before, depressed 

 behind, posterior side slightly 

 produced, rapidly narrowed, 

 angulated. 



Basal margin waved. 



Shell thick. 



Beaks elevated, recurved, pro- 

 jecting forwards nearly as far 

 as the anterior side. 



Ligament passing between the 

 beaks. 



Epidermis horn-colored, wrinkled 

 and finely striated transversely, 

 surface glabrous. 



Cardinal teeth deeply sulcated 

 and crenated. 



Trigonus. 



Subtriangular, nearly equilateral. 



Inflated, depressed before the um- 

 bonial slope, angular behind, 

 umbonial slope carinated. 



Basal margin emarginate. 

 Substance of shell thick. 

 Beaks prominent, incurved. 



Ligament short and thick. 

 Epidermis brown, rays obsolete. 



Cardinal tooth in the left valve 

 large, elevated and widely cleft. 



