41 



gently curved forward on the dorsum, and nearly straight on 

 the ventrum ; the greatest concavity being a little on the ven- 

 tral side of the centre. Seven septa measured on the back, 

 occupy a space equal to the transverse diameter. Siphuncle 

 small, dorsal. 



Surface marked by concentric lines of growth. 



This species is remarkable for the oblate, obtusely triangular 

 section, and its very gradual depression towards the aperture, 

 the amount of increase in a length of one and a half inches 

 being scarcely more than one-tenth of an inch. In two speci- 

 mens examined the septa are more distant and are not crowded 

 towards the aperture as in the preceding species, nor are they 

 bent forward on the ventral side. The siphuncle is propor- 

 tionally smaller, and the specimens preserve no markings be- 

 yond the striae of growth. 



The specimens are casts and imperfect, the largest one hav- 

 ing a diameter of three-fourths of an inch. 



Geological Formation and Locality. In the "Buff limestone" 

 of the Trenton limestone group, at Beloit, Wisconsin. 

 Collectors. T. J. Hale, James Hall. 



CYRTOCERAS fostehi, (n. s.) 



Description. Shell elongate arcuate, making less than a 

 volution, gradually tapering to the apex, sharply rounded on 

 the dorsum, and less abruptly on the ventral side, compressed 

 laterally, giving an elliptical section. Septa closely arranged, 

 somewhat deeply concave, strongly arched forward on the dor- 

 sum ; the space occupied by fine septa on the back of the shell 

 scarcely equalling the transverse diameter at the same point. 

 Siphuncle near the dorsal margin. 



Surface characters unknown. 



There are sixteen septa in the space of an inch and a half 

 from the outer chamber, the last one being a little more ap- 

 proximate. The transverse diameter near the outer chamber 

 is three-fourths of an inch, and the outer chamber, which is* 

 preserved for the length of less than an inch, continues to have 

 the same gradual expansion as the septate portion. 



This species differs from the C. Whitneyi in its more 

 gradual increase in size, in being less compressed laterally, 

 and in having the septa more regularly arranged or not in- 

 creasing in distance with the age of the shell. The septa are 

 likewise much more arched forward upon the back than in that 

 species. There arc no evidences of longitudinal striae, on the 



