136 



The surface in most of the specimens is covered with strong radiating 

 striae, which are twice and sometimes thrice divided before reaching the 

 margin. In the middle of the shell there are about six striae in the width 

 of two lines, and at the margin about four. Those at the cardiual angles 

 are sometimes a little smaller, and often a little larger than the others. 



The usual width is from 12 to 18 line's ; length about one fourth less 

 than the width. Height of the ventral area from 14 to 2 lines. 



The principal difference between this species and 0. subquadrata con- 

 sists in the more uniform and greater convexity of the dorsal valve, and in 

 the absence of a mesial fold and sinus. 



Some of the specimens are much more coarsely ribbed than others, 

 having only three ribs to two hnes at the front margin, and four or five in 

 the middle of the shell. In very perfect specimens fine concentric hnes 

 of growth are obscurely visible crossing the radiating ridges. 



Localitij and Formation. — Trenton limestone at Ottawa and near 

 L'Orignal; also at Anticosti in the base of the Anticosti group. Middle 

 Silurian. This species also occurs in the Bala group in numerous locaU- 

 ties in Wales and Ireland. 



Collectors. — E. Billings, R. Bell, J. Richardson. 



Orthis ebtrorsa. (Salter.) 



Oethis eetrorsa (Salter). Memoirs of the Geol. Surv. of Great Britain, Vol. 2, p. 373 

 PI. 27, Fig. 3, 1858. 



Orthis OAaLETi (Hall). 13th Rep. Reg. N. Y., p. 120, January 1861, dated April 10th 

 1860. 



Fig. 112. 



Fig. 113. 



Pig. 112. — Orthis retrorsa (Salter), a, dorsal, and b, side view. 



Pig. 113. — The same species from Cincinnati (0. Carleyi, Hall). It is undoubtedly 



identical with the English species, and also with the Ottawa species, only a little 



larger. 



Description. — This species only differs from 0. porcata in having the 

 area of the ventral valve inclining forward, instead of overhanging the 

 hinge-line. The specimens collected at Ottawa are much smaller than 



