186 GKETACEOUS PEltlOD. 



Family ANATINID^. 



Gekus LEIOPISTHA Meek, 18C4. 



Subgenus PSILOMYA Meek (manuscript, 1874). 



Leiopistha (Psilomya) Meekii White. 



Plate XVIII, fig. 14 a, b, e, and d. 



Leiopistha {Psilomya) Meelcii White, 1874, Exp. & Surv. west 100th Merid., Prelim. Eep. 

 Invert. Foss., 26. 



Shell short, much inflated ; iimbones large, elevated ; beaks small, 

 strongly curved inward and downward, and very slightly turned forward ; 

 posterior portion moderately produced, somewhat compressed laterally ; 

 free maa'gins forming a regular but unequally convex curve, the greatest 

 convexity of which is in front and the least along the base ; upper portion 

 of the posterior border obliquely truncated, so that the greatest posterior 

 extension of the shell is a little below the hinge-extremity. 



Surface haAdng a smooth aspect, but it is marked by fine concentric 

 lines of gi'owth. Under a lens, very fine, obscure, radiating strise are seen 

 upon the surface of a Httle more than the anterior half of the shell ; and upon 

 the remainder of the surface, except a small space adjoining the posterior 

 cardinal border, there are small, somewhat distant, radiating strise, easily 

 seen by the unassisted eye. Upon these strise, both the distinct and obscui-e, 

 the lens shows numerous minute punctures, placed at irregular intervals, 

 which are the bases of minute, short, blunt spines, or which mark the places 

 from which the spines have been removed. 



Length, twenty-five millimeters ; height, from base to umbo, twenty 

 millimeters ; greatest thickness, both valves together, sixteen millimeters. 



This shell seems to be more nearly related to L. globosa (rr Poromya 

 globosa Forbes) than to any other described species. Compared with that 

 species, as figured and described by Stoliczska (Cretaceous Fauna of South- 

 ern India, vol. iii, p. 47, pi. iii, fig. 8, and pi. xvi, fig. 16), ours difi'ers in 

 being less globular, in having the umbones more elevated, and in the more 

 distinct radiating strise upon the posterior half of each valve. 



This species belongs to an interesting group of shells, which ^ form a 

 part of the family Anatinidce, and which Mr. F. B. Meek has defined under 



