328 THE CANADIAN NATURALIST. * [Vol. vi. 



cavities have yet been observed in the dorsal valve. It thus 

 becomes evident, that the existence or non-existence of these cavi- 

 ties, is not always a character of generic value. Whether it be 

 so or not, in any particular instance, depends upon the extent to 

 which the cavities are developed. They may be so small and 

 rudimentary, as not to be even of specific value. Or they may be 

 BO large, as to constitute good sub-generic characters. I have 

 some specimens which seem to show that small cavities also exist 

 in species that, with our present knowledge, can only be referred 

 to the genus Monomer el la. In a general way, therefore, it may 

 be said that these genera are destitute of cavities, but that, ex. 

 ceptionally, they do occur, and that where such is the case, an 

 approach to the genus Trimerella is indicated. 



I consider that Obolellina, Monomerella and Trimerella, are 

 merely sub genera of a single great genus, of which the first, as it 

 is the most ancient, and the least specialized, should be regarded 

 as the type. They gradually pass into each other, and no doubt 

 as the number of species increases, it will become more and more 

 difficult to draw lines between them. 



The Canadian species are 0. Canadensis, 0. Galfensis and 

 0. mcgnijica. The second of these, has the muscular impres- 

 sions in the dorsal valve of the same form and arransemeut as 

 those of the first named. The beak of the ventral valve is very 

 large, its length being one-half that of the body of the shell. It is 

 slightly incurved. The area has three furrows, the peduncular 

 and the two lateral grooves. The muscular impressions are rhom- 

 boidal rather than ovate, and confined to the central portion of the 

 shell. There are no cavities under the area. 



I am informed that it is now proposed to place 0. Galfensis in 

 one of Prof. Hall's unpublished genera, presently to be noticed, 

 along with the species described in the 20th N. Y. Keg. Rep. p. 

 368, under the name o^Oholus Conradi. It seems to me however, 

 that this latter is a Trimerella, or rather one of those forms whose 

 position is near the dividing line between Trimerella and Obolel- 

 lina. Prof. Hall has figured the cast of the interior of a ventral 

 valve in PI. 13, fig. 2, of the work cited. Close to the area there 

 are two short obtuse cones, which are continued towards the front, 

 as two rounded ridges, one on each side of the muscular impres- 

 sions. The latter extend nearly up to the area, and are separated 

 by a small rounded ridge. These characters are all seen in the 

 cast of the ventral valve of Trimerella. They do not occur at 

 all in either of the three species of Obolellina. 



