.C ALtTTR^ID^. 107 



tiguous, without columella; aperture more or less expanded, 

 often campanulate, and sometimes with the lip reflexed ; peris- 

 tome entire or sinuous. Surface striated or cancellated, often 

 spirally ridged or plicate, and sometimes strongly lameliose 

 transversely, nodose or spiniferous. 



There are fifty fossil species, Silurian to Carboniferous. 

 United States, Europe. P. ventricosum, Conr. (S. and S. Conch., 

 t, Ixiv, f. 81, 82). As Platyceras was parasitic on crinoids and 

 other marine organisms, the shells are very frequently deformed. 

 The subglobose species resemble the Velutinse, but there is 

 every degree of variation in form between these and non-spiral 

 shells. From among these the following groups have been 

 rather arbitrarily separated. Acroculia, Phillips, 1841, is a 

 synonym ; in the opinion of some naturalists, Conrad's name 

 (being preoccupied in insecta) should yield to this. 



Section Orthonychia, Hall, 1843. 



Body of the shell straight or curving, gradually diminishing 

 above, arched or in some degree spiral at the apex, with the 

 last volution or more quite free. F. spirnle, Hall (S. and S. 

 Conch., t. Ixiv, f. 83). 



Section Igoceras, Hall, 1859. 



Shell cancellated and often plicate. P. pileatum, Conr. Si- 

 lurian, U. S. 



Section Exogyroceras, Meek and Worthen, 1868. 



Shell sinistrally spiral, with rudimentary columella. P. 

 r ever sum, Hall. 



[Genus DIAPHOROSTOMA, Fischer, 1885. 



Under this new name. Dr. Fischer includes Platyostoma, 

 Conrad (not Klein, etc.), and its section Strophosti/lus in this 

 subfamily. I have placed it in Naticidge, p. 9.] 



[ADDISONIA, Dall, 1882. 



Dr. Fischer has included this patelliform genus here on 

 account of its lingual dentition, and the existence of a male 

 copulatory organ ; for botli conchological and anatomical reasons 

 I prefer to retain it near Fissurella,] 



