39 



back, the ventral sides being slightly concave, giving a very 

 obtusely quadrangular section to the volution, the lateral di- 

 ameter of which is a little greater than the dorso-ventral di- 

 ameter. 



Septa concave, numerous, these being on the back, about six 

 in the space equal to the lateral diameter at the same point. 

 Siphuncle small, situated on the back of the volution, outer 

 chamber very gradually expanding. 



Surface marked by obscure undulating folds, which com- 

 mence upon the vertical side of the volution, and arching back- 

 wards unite with those from the opposite side in low ridges 

 which are bent backwards in a broad sinus upon the dorsum. 

 Lines of growth parallel to the undulations cover the entire 

 surface. 



This species differs conspicuously from L. undaius of the 

 Trenton limestone of New York in the much more closely ar- 

 ranged septa, those of that species being less than half the 

 number in the same space where the diameter of the volution 

 is the same. 



Geological Formation and Locality . In the " Buff limestone' 7 

 of the Trenton limestone group, at Beloit, Wisconsin, and 

 Rockford Illinois. 



The species is dedicated to T. D. Robertson, Esq., of Rock- 

 ford. 



GENUS CYRTOCERAS, goldfi 

 CYRTOCERAS wiiitneyi, (n. s.) 



Description. Shell robust, elongate arcuate, somewhat rap- 

 idly tapering, laterally compressed, giving an eliptical section, 

 the lateral diameter being equal to three-fourths the ventral 

 diameter, outer chamber short, septa numerous, but little con- 

 cave, and exteriorly are directed forward on the dorsum, more 

 closely arranged towards the apex and generally becoming more 

 distant as the shell expands towards the aperture. On one 

 specimen at a point where the dorso-ventral diameter measures 

 three-fourths of an inch there are twelve septa in the length 

 of one inch on the dorsum, while in the outer part where the 

 diameter is less than one inch and one-fourth, there are but six 

 septa in the length of one inch. 



The surface of the shell is exfoliated, but there are distinct 

 traces of longitudinal ridges which are situated at about three 

 times their diameter from each other. In another speimen 

 preserving a portion of the shell, fine concentric lines of growth 

 with more closely arranged longitudal ridges are visible. 



