CRANIO OF THE HELDERBERG AND HAMILTON GROUPS. 97 



CRANIO OF THE UPPER HELDERBERG GROUP. 



Crania aurora ( n. s.). 



PLATE III. 

 A cast of Trochoceras, from the Schoharie grit, preserves the impres- 

 sions of two individuals of a species of Crania, which were apparently- 

 adhering to the interior of the shell of the outer chamber of this cepha- 

 lopod. 



Ventral valve subquadrate, approaching a circular form ; lateral margins 

 slightly curved, and the two extremities abruptly rounded, the posterior 

 one least curved : margins thickened. The impressions of the anterior 

 muscles are nearly united in a transversely subelliptical scar : posterior 

 adductor scars distant, not well defined in the specimens. 

 No other specimens of this or any other species of the genus are known to me, 

 at this time, from this formation. 



Geological formation and locality. In the Schoharie grit, in the town of Knox, 

 Albany county. 



CRANIO OF THE HAMILTON GROUP. 



Crania hamiltonite. 



PLATE III. 

 Crania hamiltoniiB : Hall, Thirteenth Report on the State Cabinet, p. 77; and f. 4 & 5, p. 76. 



Shell broadly oval or subcircular. Dorsal valve subconical ; apex sub- 

 central or excentric, pointed in well-preserved specimens, often worn 

 or decorticated. Exterior surface of dorsal valve marked by concentric 

 lamellose striae. Ventral or lower valve marked by four strong impres- 

 sions of the adductor muscles, which are variable in form : the poste- 

 rior ones are distant ; the anterior ones approximate, diverging above 

 and assuming a somewhat cordiform appearance, the pit for the pro- 

 tractor muscles occupying the space between. Vascular impressions 

 strongly digitate. 



