GENUS CYRTOCERAS. 357 



piisillu?n), and a Trochoceras ; and is not an abundant form, so far as 

 known. 



Formation and locality. — In limestone of the age of the Niagara group, 

 at Racine, Wisconsin. 



CYRTOCERAS PUSILLUM (n.s). 



Shell small, slender, making half a volution, somewhat rapidly 

 expanding near the aperture ; section broadly elliptical ; the length of 

 the chamber of habitation, as preserved, about equal to once and a half 

 its lesser diameter. Septa moderately concave, arching forward on the 

 back so as to give a depth once and a half as great as on the sides. 

 Siphuncle dorsal. Surface unknown. 

 Formation and locality. In limestone of the age of the Niagara group, 



at Racine, Wisconsin. 



CYRTOCERAS LATERALS (n.s.). 



PLATE 18 (9), FIGS. 4-6. 



Shell rather above the medium size, very gently curving, and the sides 

 abruptly expanding above the middle, the ventral line being nearly 

 straight, dorsal line more strongly arcuate : shell more rapidly expand- 

 ing transversely than in the opposite direction, giving to the middle of 

 the shell a broad and somewhat flattened appearance. The outer chamber 

 gently converges again above the last septum, and is broadly constricted 

 below the aperture, where it is nearly straight for a short distance. 

 Section transversely oval, and a little flattened on the ventral border 

 in the middle portion of the shell, while in the lower or smaller part it 

 is circular. Septa distant about one-sixth the transverse diameter of the 

 shell, moderately concave, their margins directed slightly upwards on 

 the lateral portions of the shell, giving a broad shallow sinus on the 

 dorsal and ventral sides. Siphuncle small, situated near the dorsal mar- 

 gin. Surface of the shell marked by obscure longitudinal ridges, distant 

 from each other from a sixteenth to a tenth of an inch in diff'erent indi- 

 viduals and on diff'erent parts of the shell. The impressions of these 

 ridges are distinctly seen oh well-preserved casts of the interior. 

 This species is readily distinguished from the others of the genus asso- 

 ciated with it, by the slight curvature and the greater transeverse diameter ; 

 the relative diameters of the specimen figured, in its larger parts, being as 

 four to five. The relative distance of the septa may vary somewhat in 

 diff'erent individuals, judging from other specimens before me, but not 

 materially affecting characteristic features of the species. The outer two 

 septa, next the chamber of habitation, are often crowded close together. 



Geological formation and locality. In the limestones of Niagara age, 

 at Racine, Wisconsin. 



