OF CENTRAL CANADA PART IV. 



271 



Fig. 190. Rhynconella plena, Chazy 

 Formation. 



Fig. 191. R. increbescens, Trenton 

 Formation. 



Fig. 192. R. (Camerella) varians, 

 Chazy Formation. 



Fig. 193. Pentamerus oblongus, Clin- 

 ton and Niagara Formations. 



Family 5. Strophomenidce : Shell, 

 mostly, with one valve convex and the 

 other flat or concave, but in some forms 

 193 biconvex. No internal arm-supports. 



Hinge-line straight. 



This family is entirely extinct and almost exclusively palaeozoic. 

 Its more characteristic genera comprize : (1) Strophomena, with 

 concavo-convex or plano-convex shell and greatest width at hinge- 

 line : the four most distinct muscular impressions in a single row : 

 Silurian to Carboniferous period : (2) Lepto&na, much like strop- 

 homena but muscular impressions very long : Silurian to Liassic 

 period. (3) Orthis, shell flatly bi-convex or plano-convex, with 

 greatest width below the hinge-line ; the four most distinct muscular 

 impressions not in a single horizontal row : Cambrian to Carbon- 

 iferous. (4) Platystrophia, with strongly bi-convex shell : one valve 

 depressed in the centre and the other with mesial fold, thus greatly 

 resembling a spirifer shell. Surface of shell strongly ribbed : Silurian 

 to Carboniferous. Representatives of these genera are shewn in the 

 following figures. 



194. 196. 197. 



Fig. 194. Strophomena alternata : Trenton and Hudson River 

 Formations. 



Fig. 196. Leptcena sericea: Trenton and Hudson River Formations. 



Fig. 197. Orthis testudinaria : Trenton and Hudson River Forms. 

 0. elegantula, an Upper Silurian form, much resemble this species. 



