CANADIAN PERIOD. 53 



and sharper, and the postero-lateral slopes straighter than they are in the 

 dorsal valve. Surface of both valves having a smooth appearance, but fine 

 concentric lines and obscure radiating striae are to be seen under a lens. 



Length of the dorsal valve represented by figure 2 «, Plate III, six 

 millimeters '; width, four millimeters. This is the largest valve the collec- 

 tions contain, but it is not improbable that the species attains a larger size. 



This species in general aspect resembles Lingula acuminata Coiu-ad, from 

 the Calciferous sandstone formation of New York, especially in the narrow- 

 ness of the posterior portion of the ventral valve and its slender beak ; but 

 it is not proportionally so broad anteriorly as that shell is, and in other 

 respects the outline is materially different. The dorsal valve of our shell 

 resembles the figure of a specimen that Professor Hall refers doubtiugly to 

 Lingula mosia, from the Potsdam foianation of Wisconsin (Sixteenth Ann. 

 Regent's Report New York State Cabinet, plate 6, figure 1); biit the ventral 

 valve of ours is proportionally longer than that figure, which is vmderstood 

 to represent a ventral valve. If this supposition is correct, that species has 

 proportions materially different from those of ours. 



I refer this species with doubt to the genus Lingula, because the inter- 

 nal characters of the shell are unknown, and because it is now generally 

 admitted that among the linguloid shells of the older Paleozoic rocks, which 

 were formerly referred to the genus Lingula without question, there are 

 really several different genera, all distinct from the recent Lingula. 



Position and locality. — Strata of the age of the Quebec group of Canada; 

 Schellbom-ne, Schell Creek range, Nevada. 



Family DISCINIDiE. 



Genus ACROTKETA Kutorga, 1848. 



Acrotreta pyxidicula White. 



Plate III, tig. 3 a and h. 



Acrotreta pyxidicula y^hita, 1874, Exp. & Surv. westlOOtb Merid., Prelim. Rep. Invert. 

 Foss., 9. 



Shell minute ; marginal outline subcircular or transversely suboval ; 

 without observable mesial sinus or fold. Dorsal valve most prominent near 

 the umbo ; beak small, depressed, but well defined, hardly projecting 



