138 



margin ; gradually becoming finer towards the cardinal angles, one half 

 running all the way to the beak, and the other half dying out at from a 

 half to two thirds the distance. Width of average-sized specimen 9 lines ; 

 length 6 lines ; height of area of ventral valve 1 J lines. 



The front margin is usually straight in the middle, but sometimes it is 

 rounded as in the specimen above figured. 



Locality and Formation. Gramache Bay, Anticosti. In Division 1, 

 Anticosti group ; Middle Silurian. 



Collector. J. Richardson. 



ORTHIS LAURENTINA. (Billings.) 



OETHIS LAURENTINA (Billings). Report G. S. C. for 1857, p. 297. 



6 



Fig. 115. 

 Fig. 115. 0. Laurentina. a, ventral valve ; b, area ; c, side view. 



Description. Semielliptical, broader than long in the proportion of 

 about seven to five ; hinge-line straight, slightly exceeding the width of 

 the shell ; the dorsal valve flat, very slightly convex, the most elevated 

 point being at the beak, a perceptible depression along the middle ; car- 

 dinal area low, triangular, inclining backwards at an angle of 100, or a 

 little more, to the plane of the lateral margin ; foramen partly closed. 

 Ventral valve convex, most elevated at one third the length from the 

 beak, which is small, pointed and depressed about one third below the 

 greatest height of the valve ; cardinal area rather large, triangular, some- 

 what concave, extending to the beak, but closed by a convex deltidium 

 all but a small triangular space at the base. Surface with from twenty 

 to twenty-five prominent, subangular radiating ridges, which gradually 

 enlarge from the beak to the base, separated by the same number of 

 sulci equal to the ridges in breadth and depth, two ridges in one line at 

 the front margin. Some of the specimens are obscurely subquadrate. 

 Width of large specimen seven lines ; length five lines ; height of ventral 

 area at the beak 1 J lines ; width of foramen at the base of a line. 



This species is closely allied to both 0. tricenaria and 0. disparalis. 

 Indeed these three would be by some good naturalists all united. The 

 only differences that I can perceive are that this species is smaller than 



