596 



THE UPPER SILURIAN. 



ventral valve variably convex, and often showing a flattened or 

 slightly concave space down the middle of the shell ; cardinal margin 

 ornamented by four or five minute spines on each side of the beak ; 

 cardino-lateral margins often a little wrinkled ; surface finely striated, 

 striae flexuous, dichotomising and increasing by interstitial addition, 

 so that there are more than 100 on the margin of the shell ; striae 

 increasing in size below the umbo ; concentric striae fine, close, 

 rounded and slightly undulating. 



Fig. 198. 

 Crania Acadiensis. 



Fig. 201. 

 Trematospira Acad'ue. 



Fig. 199. 



Chonetes Nova Scotica, and 



portions magnified. 



Fig. 200. 

 Chonetes tenuistriata. 



Fig. 202. 

 Leptocoelia intermedia. 



Dorsal valve moderately concave ; striae much stronger below the 

 middle of the shell and sometimes bifurcating toward the margin. 



This species resembles in form the Chonetes cornuta of the Clinton 

 group of New York, but is a much larger and more ventricose shell ; 

 the striae are proportionally less numerous and more closely arranged, 

 the interstices being less than the striae, while in the C. cornuta the 

 interstices are wider than the striae, and the latter increase only by 

 interstitial additions below the middle of the shell. A stronger and 

 more elevated stria often marks the median line from beak to base of 

 the ventral valve. Arisaig, East River, Nictaux, coll. J. W. D. 



Chonetes tenuistriata, Hall (Fig. 200). Shell semi-oval, twice as 

 wide as long ; ventral valve moderately convex, hinge-line equalling 

 the width of the shell ; surface marked by fine, even, closely arranged 

 striae, which apparently increase only by interstitial additions, and are 

 not flexuous. The number of striae on the margin of the shell is 

 nearly 100. 



This species is more finely striated than the preceding, the striae 

 not flexuous, more even, and in shells of equal size much more 

 numerous. This species is sowewhat larger and more closely striated 

 than the C. cornuta of the Clinton group of New York. Arisaig, East 

 River, coll. J. W. D. 



Spirifer rugcecosta, Hall. Shell somewhat semi-elliptical ; dorsal 

 valve very convex, with the mesial fold depressed along the centre ; 

 ventral valve with a wide deep mesial sinus ; plications six or seven 





