MINERAL.5 AND GEOLOGY OF CANADA. 



113 



Fig. 98, 



Pig. 99. 



mucronattis of the Devonian series. Both are of common occurrence. 

 The hinge-line of the latter is sometimes shorter (as compared with 

 the height of the shell,) than is shewn in the figure. In our Western 

 Devonian rocks, several other species occur : as S. duodenarius, with 

 eight or nine rounded ribs on each side of the mesial fold ; S. rari- 

 costatus, with two or three coarse plications on each side of the fold ; 

 S. gregaria, a small species, &c. These are figured and described by 

 Mr. Billings in the Canadian Journal, vol. YI. Another common 

 species of the Upper Silurian series, is S. radiatus. This differs 

 chiefly from S. Niagarensis by its finer and more numerous plications. 

 A third Niagara species S. sulcatus, has about eight plications on 

 each side of the mesial fold, crossed by the rough and strougly-pro- 

 nounced edges of the layers of growth. 



Aihyris : — The shell in this genus has internal spires as in Spirifer, 

 but the hinge-line is curved, and the area is absent or rudimentary. 

 Species range from the Silurian to the Triassic formations. Several 

 occur in our Devonian rocks. One of the most common of these. 



<:'T^ 



Fig. 100 



A. clara, (Billings,) is represented in fig. 100. 

 A. Maia is a somewhat similar species, but with 

 a more developed or longer mesial fold and 

 sinus, and with a slight space or false area 

 between the beaks. These and other Devonian 

 species are described in detail by Mr. Billings, 

 in the Canadian Journal, Vol. V. 



Spirigera : — This genus or sub-genus differs^from Aihyris in hav- 

 ing a perforation or foramen in the beak of 

 the ventral valve. S. concentrica of the 

 Devonian rocks is shewn in fig. 101. The 

 genus Betzia is nearly allied to Spirigera, 

 but the shells are smaller and strongly ribbed. 



Vol. VII. 



Pig, 101. 



