No. 2.J BILLINGS — PALEOZOIC FOSSILS. 221 



venturiug to describe it, I sent a specimen to Mr. Davidson, and 

 on returning it he stated that he considered it to be a new genus, 

 " very closely allied to Trhnerdla.'' Lately I received a letter 

 from him in which he states that he has obtained the same genus 

 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 charac- 

 terize our species briefly as follows. Full descriptions and figures 

 will be given hereafter. 



M. PRISCA. — Ventral 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. Dorsal valve 

 about one-fourth shorter than the ventral, and more broadly 

 rounded at the anterior extremity. On a side view the outline 

 of the ventral valve would be, so far as we can iudoe from a cast 

 of the interior, somewhat straight, or only gently arched from the 

 beak to the front margin. The dorsal valve, on the other hand, 

 is rather strongly convex, most prominent in the anterior half. 

 It is evident that the general cavity of the shell of the dorsal 

 valve extends a short distance under the area. 



Length of ventral valve, eighteen lines ; greatest width, thirteen 

 or fourteen lines ; length of dorsal valve about fourteen lines. 

 There are some fragments in the collection which indicate a larger 

 size. 



Occurs in tlie Guelph limestone at Ilespelar, Ontario. Col- 

 lected by T. C. Weston. 



M. 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- 

 wards a short distance from the cardinal edge, supported by the 

 septum. The length and width appear to be about twelve or 

 fifteen lines. 



Occurs with M. j^risca. T. C. Weston, collector. 



Both Trimerella and Monomerella are sub-genera of Obolus. 



There is, besides the above, a third group which differs from 

 the other two in having no cavitias in either valve. It includes 



