182 



FIG. 

 I 



NEW YORK STATE MUSEUM 

 PLATE 20 



Trochoceras costatum Hall 



Page 103 



Specimen showing the rate of expansion, curvature of conch 

 and the character of the costae. From a gutta-percha 

 impression of a natural mold 



An individual which shows the living chamber and the curva- 

 ture and depth of the last septum. The costae on the 

 living chamber are drawn somewhat too strong. 



Trochoceras desplainense McChesney 



Page 100 



Enlargement of surface (X3) to show the fine transverse and 

 longitudinal lineation. From a gutta-percha impression of 

 a natural mold 



Internal cast of specimen, showing the aperture, living cham- 

 ber and septal sutures of an early whorl 



Specimen with strongly developed costae and growth lines 



