Cephalopoda of the Cincinnati Group. 131 



Endoceras approximatum(?) — (Hall, 1847). 



Cylindrical, gradually tapering; septa ^vith a convexity little more 

 than one fourth their diameter, distant about one fifth the diameter of 

 the shell ; siphuncle large, marginal, obliquely annulated by the thin 

 edges of the septa. 



The specimen doubtfully referred to this species I found at Rich- 

 mond, Indiana. Only part of two chambers are preserved. The 

 siphuncle is two inches in length, and one half inch in diameter, and 

 marked by the edges of the septa, with nine strongly oblique annula- 

 tions. It will be observed, that the proportions of the specimen do not 

 exactly correspond with the definition of the species; but the 

 siphuncle is almost cylindrical, tapering but very little, and the 

 annulations being quite oblique, it must be very closely related to this 

 species, if not identical. 



Family Cyrtoceratidce. 



Shell more or less curved on the same plane ; siphuncle varying in 

 position ; section circular, or sometimes having a transverse or longi- 

 tudinal diameter the greater ; body chamber sometimes expanded, and 

 at other times contracted anteriorly. 



Genus Cyrtoceras — (GoLDFUSS, 1833). 



\_Etym. — Kurlos, curved ; keras, horn.] 



Shell curved, or partially involute, sometimes with the transverse, 

 at others the longitudinal diameter the greater ; aperture sometimes 

 contracted ; siphuncle varying from the center to the concave or 

 convex side. 



Cyrtoceras vcntricosa — (S. A. Miller). 



Shell short, veutricose, gently curved, and very rapidly enlargiag 

 toAvard the aperture ; section circular or nearly so ; septa slightly 

 concave, and curving forward on the dorsal side, over the siphuncle. 

 Siphuncle close to the shell on the dorsal side, and abruptly expanded 

 Avithin the chambers, forming a cylindrical tube, having a diameter 

 greater than the distance between the septa, and about five times as great 

 as the small circular aperture piercing the septa, through which it passes. 



Length of specimen, 2.30 inches; diameter at the small end, 0.44 

 inch, and at the distance of 0.80 inch from the small end, 0.70 inch. 



