Contributions to Invertebrate Palaeontology. 533 



in a transverse section, being a very little greater in the lateral direction than 

 in the dorso-veutral, and the back of the volution barely perceptibly flattened 

 on the outer portion of the larger one, but not perceptibly so on the inner 

 portions. Septa deeply concave and distantly arranged ; the chambers mea- 

 suring about half an inch each, on the outer two-thirds of the body volution 

 of a specimen where the vertical, or largest, diameter of the disk is five inches. 

 Position of the siphuncle not absolutely determined. Surface of the shell 

 unknown. 



All the individuals of this species observed are internal casts, and 

 occur in a rather rotten limestone, under conditions very unfavor- 

 able for the preservation of the shelly substance ; consequently 

 the surface characters have not been observed. It is an abundant 

 species, but owing to the conditions of preservation, is not often 

 found in collections. It will be readily distinguished from the 

 other described species by the closely coiled volutions and the 

 nearly circular section. It is perhaps more nearly related to G. 

 Cyclops Hall, 15th Kept. N. Y. State Cab. Nat. Hist., than to any 

 other described species ; but it differs from that one in its smaller 

 size, and more rapidly increasing as well as more closely coiled 

 volutions, and does not appear to have been provided with the 

 broadly expanding and foliated varices which are so characteristic 

 of that species. It might be objected, that as the shell of this 

 species is unknown, the determination of the absence of these 

 foliated expansions is not well authenticated ; but it may be 

 answered, that as the two species are associated in the same layers 

 in the quarries where they are both rather common, if they were 

 really one and the same, the shell would be preserved on these as 

 well as on the G. cyclops, and the expansions readily detected. 



Formation and Locality. — In the limestone of the Upper Helder- 

 berg group, near the lower part, at Smith and Price's and at other 

 quarries near Columbus, Ohio. 



Gyroceras seminodosiim. 



Plate VIII, fig. 5. 



Gyroceras seminodosuni Whitf., Ann. N. Y. Acad. Sci., March, 1882, p. 211. 



Shell small, compactly coiled, and consisting, in the specimen used, of a 

 little more than two volutions, which increase rather rapidly in diameter with 

 increased age ; they are somewhat wider transversely than in a dorso-ventral 

 direction, and are slightly triangularly elliptical in a transverse section ; the 

 greatest transverse diameter being very slightly outside of the middle of the 

 dorso-ventral diameter. The inner one and a half coils are smooth on the 



