43 



equal to the transverse diameter of the specimen at the same 

 point; having a broad, shallow, retral undulation on the dorsal 

 side. Siphuncle dorsal small where it passes through the 

 septa, and enlarging within the chambers. 



Surface longitudinally striated. 



This species is readily recognized, and distinguished from 

 any others in the same association by its great expansion, and 

 by its distant septa, as well as in having the transverse diam- 

 ter greater than the dorso-ventral diameter. Length from 

 four to six inches or more. 



Geological Formation and Locality. In limestone of the age 

 of the Niagara group, Racine, and at Waukesha, Wisconsin, 



CYRTOOERAS dardanum, (n. s.) 



Description Shell robust, strongly curved, moderately ex- 

 panding from the apex, and slightly contracting near the 

 aperture; transverse section broadly elliptical, the greater 

 diameter in a dorso-ventral diameter. Dorsal and ventral 

 sides equally rounded. Septa distant, measuring 6nly four in 

 a distance equal to their transverse diameter, deeply concave, 

 and strongly arched forward on the dorsum. The siphuncle of 

 moderate size, dorsal. 



Surface apparently smooth or with only lines of growth, the 

 small remains of the shell showing no markings. The length 

 of entire individuals has been from six to eight inches. 



The specimens examined are casts of imperfect individuals, 

 but they are so well marked as to be easily recognized. 



Geological formation and locality. In limestone of the age 

 of the Niagara group, at Waukesha, Wisconsin. 



GENUS ONCOOERAS, hall. 



in the first volume of the Palalontology of New York, I 

 have proposed a separation of the forms like Cyrtoceras, when 

 the aperture is abruptly constricted, and the apex more ab- 

 ruptly tapering from the middle of the shell. Some of the 

 forms are much like curved Gomphocwas, with a more extended 

 apex. The section is usually broadly elliptical and the 

 siphuncle dorsal, or on the outer side of the curve. 



The contraction towards the aperture is often extreme, and 

 in all individuals which I have seen, this feature is strongly 

 marked. The genus is at least convenient for the reference of 

 species of this peculiar type. 



