270 



MINERALS AFD GEOLOGY 



Family 2. Atrypidce, : Shell with internal spires attached to th 

 hinge-line of the dorsal valve. No area. Shell bi- convex. 



This family differs from that of the Spiriferidse 

 by the direction of the spiral supports. The only 

 typical genus Atrypa ( = Spirigerina) much re- 

 sembles the spirifers with curved hinge-line. Our 

 most common species is A. reticularis, fig. 188, 

 an Upper Silurian and Devonian type of wide 



*Y. Fro. 188. 



geographical range. Atrypa reticularig 



Upper Silurian. 



Family 3. Terebratulidce : Shell with both 

 valves convex. Internal arm-supports in the form of a short or long 

 calcareous loop. 



This family has several living representatives, notably, Terebratula, 

 dating from the Devonian period, and Waldheimia, from the Triassic, 

 In the former, the internal loop is short, and in the latter compara- 

 tively long and deeply recurved. The genus Centronella, an extinct 

 Devonian type is the principal if not the only Canadian representa- 

 tive of the family. Fig. 189 represents C. 

 glans-fagea of Hall. 



Family 4. Rhynconellidce : Shell wit* 1 

 both valves convex; usually strongly 

 ribbed. Internal arm-supports in the form 

 of very short, more or less inconspicuous 

 points. 



This family includes the typical genus 

 Rhynconella, dating from the Silurian 



period and still surviving, and the extinct genera Pentamerus (Sil- 

 urian to Carb.), Stricklandia, and Camerella : the two latter, Silurian 

 only. Several species of Rhynconella and a common Pentamerus 

 are represented in figures 190 to 193. 



FIG. 189. 

 Centronella-glans fagea. 



(Hall.) 

 Devonian. 



1)0. 



191. 



