% 
E. Billings—New Species of Paleozoic Fossils. 859 
from Wisby, Island of Gothland, and he requested me to name 
it, as he was about to publish the Swedish species. 
We have two distinct species, both occurring in the Guelph 
limestone. This formation I consider to be about the age of 
the Aymestry limestone of the English geologists. I shall char- 
acterize our species briefly as follows. Full descriptions and 
figures will be given hereafter. 
M. prisca.—V entral valve ovate, greatest width at about the 
anterior third of the length, thence tapering with gently convex 
sides to the narrowly rounded beak; front margin broadly 
rounded ; septum about one-third the length of the shell. Dor- 
sal valve about one-fourth shorter than the ventral, and more 
broadly rounded at the anterior extremity. On aside view the 
outline of the ventral valve would be, so far as we can judge 
Length of ventral valve, eighteen lines; greatest width, thir- 
teen or fourteen lines; length of dorsal valve about fourteen 
lines. There are some fragments in the collection which indi- 
cate a larger size. 
Occurs in the Guelph limestone at Hespelar, Ontario. Col- 
lected by T. C. Weston. : 
- orbicularis.—Broadly ovate, nearly circular, lenticular, 
both valves moderately convex; septum about one-third the 
length. The casts seem to show that a thin plate extends for- 
ward a short distance from the cardinal edge, supported by the 
re sai The length and width appear to be about twelve or 
n lines, | 
Occurs with M. prisca. 'T. C. Weston, collector. 
Both Trimerella and Monomerella are sub-genera of Obolus. 
There is, beside the above, a third group which differs from 
the other two in having no cavities in either valve.* It includes 
the species Ihave called Obolus Canadensis and 0. Galtensis. 
For this group I would propose the name OBOLELLINA. It 
* Since the above was A : F I have ascer- 
: ublished in the Can. Nat., in Dee. last, ve 
‘ained that cavities may sds both in Monomerella and Obolellina. Where they do 
maces however, in species of these genera, they are small or rudimentary as com- 
bad their great size in Trimerella. y r in | 
Prepac oe not sc lyonae When, therefore, they are ae 4 slightly es 
. », lot even of specific value. But when very ey may 
generic importance. . y large, 
* 
