218 THE CANADIAN NATURALIST. [Vol. vi. 



corresponding to those of the ventral valve, and sometimes an 

 obscure rounded ridsre alono; the median line. 



If we compare the interior of the ventral valve of an OhoJella 

 with that of Obolus AjjoUinis, we see that there are six muscular 

 impressions in each, but not arranged in the same manner. The 

 two small scars aa at the hinge line are most probably the same 

 in both 2;enera. The two lateral scars hb of Obolus have no 

 homoiogue in Obolella, unless they be represented by the two 

 large ones dd. Should this be the case, however, the great diffe- 

 rence in their position, would no doubt be of generic value. I 

 think it more probable that the large scars dd of Obolella repre- 

 present the central pair cc of Obolus. Again, Eichwald says that 

 in the interior of the ventral valve of 0. Aj)ollonis there is a 

 longitudinal septum (shown in the above fig. 7 at s), w^hich sepa- 

 rates the two adductors cc, and extends to the cardinal groove 

 (I suppose he means the groove g on the area).* No such sep- 

 tum occurs in any species of Obolella. I have not seen any de- 

 scription of the dorsal valve of the 0. Appollinis sufiiciently 

 perfect to afford a means of comparison with that of Obolella^ 

 but the differences in the ventral valve alone are so great that the 

 two genera can scarcely be identical. They are, however, closely 

 related, and occur in nearly the same geological horizon. 



In the rocks below Quebec and at the Straits of Belle Isle, we 

 find the following species of _Obok'lla : — 



1. 0. desquamata, Hall, .- Avicula f desquamafa, Pal. N.Y., 

 vol. 1, p. 292, pi. 80, fig. 2. Occurs at Troy, N. Y. . 



2. 0. crassa, Hall = Orbicula f cnissa, op. cit. p. 299, pi. 79, 

 fig. 8. Occurs at Troy. 



3. 0. coelata^ Hall, = Orbicula coslafa, op. cit. p. 290, pi. 79, 

 fig. 9. Occurs at Troy. 



4. 0. gemma, n. sp. 



5. 0. Circe, n. sp. 



6. 0. chroma fieri, Billings, has been found as yet only at the 

 Straits of Belle Isle. 



The following are new species : 



0. GEMMA. — Shell very small, about two or three lines in 

 length, ovate, both valves moderately convex and nearly smooth. 



* Speakini;- of the adductors, he Sciys : " Une crute longitudinal 

 occupe le milieu des dernieres impressions ct arrive jusqu'au sillon 

 cardinal.'' (Lelhea Rossica, vol. 1, p. 925.) 



