282 



MINERALS AND GEOLOGY 



223. 



Fig. i!9. Pleurotomaria Progne (Billings). Lower Trenton for- 

 mation. 



Fig. 220. Murchisonia gracilis. Trenton and Hudson River for- 

 mations. 



Fig. 220a. Cast of M. gracilis. 



Fig. 221. Bellerophon bilobatus. Trenton and Hudson River for- 

 mations. 



Fig. 222. Maclurea Logani (shell and operculum). Lower Tren- 

 ton formation. 



Fig. 223. Platyceras angulatum. Trenton formation. Other 

 species are without the strong ribs on the shell ; and in some, the 

 shell is spine-bearing. 



Fig. 224. Holopea Guelphensis. Upper Silurian. 



Fig. 2-^. Subulites elonyatus (Internal cast) : Lower Silurian. 



These and other species of gasteropods occur very commonly in the 

 fo-m of internal casts, and in that condition their specific distinctions 

 are not, in all cases, easily determined. Many so-called species, 



