MINERALS AND GEOLOGY OF CANADA. 



115 



Pentamerus : — In this genus, the shell is prominently bi-convex, 

 with arched hinge-line and large incurved beak. Internally it is 

 divided by septa into several chambers. The genus ranges from the 

 Silurian to the Carboniferous formations. P. oblongus, of the Nia- 

 gara Group, is represented in fig. 107, the sketch 107 a shewing a 

 ventral view of the internal cast. P. aratus, of the Devonian 

 rocks, is figured in 108. This latter form is closely related to the 

 well-known typical species P. galeatus. 



Fig. 108. 



Fig. 107. 



Pig. 107 o. 

 Stricklandia : — This genus has been recently 

 established by Mr. Billings. It includes cer- 

 tain more or less oval forms with nearly equal 

 valves, formerly referred to Pentamerus, S. 

 elongafa, a Devonian species, is shewn in fig. 109. 

 Other species occur in these and in the Upper 

 Silurian rocks. 



Our Canadian formations do not appear, as 

 yet, to have offered any examples of the well- 

 known genera, Crania, Calceola, and Terrebra- 

 tula. 



Lambllibeanchiata (or Conchifeea.) — Lamellibranchiate mol- 

 lusks are marine or fresh-water animals of the acephalous type. 

 They are provided in the adult condition with laminated gills or 

 branchiae for breathing purpose?, (as seen, for example, in the so- 

 called " beard " of the oyster,) and they secrete a bi-valve external 

 shell. The two valves in most genera (those of the Ostreidce and 

 some Aviculidce are the only exceptions) are of equal size, but 

 always more or less inequilateral. (See under the Braehiopods 

 above). These raollusks are exceedingly abundant in the fossil state, 

 though less numerous than the braehiopods in the older rock for- 



Fig. 109. 



