-406 REPORT ON THE STATE CABINET. 



in the greater length and more extreme constriction of the intermediate 

 portion of the aperture. 



This species should be compared with P. ventricosum and P. arcuatum, 

 MuECHisoN, Silurian System and Silmia. 



Formation and Locality. — In limestone of the Niagara group, at Wau- 

 watosa, Wisconsin. 



GENUS CYRTOCERAS, Goldfuss. 

 Cyrtoceras lucillum, n. s. 



PLATE XVIII, FIG. 7. 



Shell arcuate, gradually expanding towards the aperture, section ellipti- 

 cal, a little narrower on the inner side of the curve ; septa mode- 

 rately convex, closely arranged; siphuncle small, submarginal on 

 the outer side of the arch. Surface marked by regular, equal, neatly 

 defined annulations, which have a slight retral arch on the exterior 

 curve, indicating a similar sinuosity in the margin of the aperture. 

 The annulations are closely arranged ; on the smaller parts of the 

 shell, they are in the proportion of twenty in the space of an inch • 

 while on the inner side of the curve they are more approximate, 

 and on the outer side more distant. On the larger part of the shell 

 the annulations number five or six in the space occupied by seven 

 in the smaller parts. 

 Formation and Locality. — In limestone of the Niagara group near Wau- 



watosa, Wisconsin. 



» 



Cyrtoceras fosteri, Hall. 



PLATE XVI, FIGS. 11, 12, 13. 



Cyrtoceras fosteri, Hall. Report Progress GeoL Survey Wisconsin for 1860, p. 43. 1861. 



This species is known to me only in the Niagara limestone near Chicago. 

 Cyrtoceras dakdanus, Hall. 



PLATE XVII, FIGS. 3, 4, 5. 

 Cyrtoceras dardanus, Hall. Report Progress Geol. Survey Wisconsin for 1860, p. 43. 1861. 



This species was originally described from fragments, which in all the 

 specimens seen can be readily identified. Some other imperfect speci- 

 mens which have come under my notice since the publication of the 

 original description, have induced me to suppose that the fossil may 

 belong more properly to the Genus Gyroceras. The species occurs at 

 Waukesha and Wauwatosa, Wisconsin. 



