26 



Greatest length of a fair average specimen, from the umbo to the 

 postero-ventral margin, twenty-six millimeters ; thickness, sixteen mil- 

 limeters. 



This species is distinguished from others of the genus by its small size, 

 prominent, pointed beaks, and subequal valves. From the young of I. 

 problematicus, it is distinguished by the usual characteristics of mature 

 growth. 



Position and locality. Strata of Cretaceous age, Ojo del Pescado, New 

 Mexico. 



Genus LEIOPISTHA Meek. 

 Subgenus Psilomya Meek. 



LEIOPISTHA (PSILOMYA) MEEKII (sp. nov.) Shell short, much inflated; 

 umbones large, elevated; beaks small, strongly curved inward and down- 

 ward, and very slightly turned forward ; posterior portion moderately 

 produced, somewhat compressed laterally; the free margins forming a 

 regular but unequally convex curve, the greatest convexity being in 

 front, and the least along the base ; upper portion of the posterior bor- 

 der obliquely truncated, so that the greatest posterior extension of the 

 shell is a little below the hinge-extremity. 



Surface smooth in general aspect, but it is marked by fine-concentric 

 lines of growth. Under a lens, very fine, obscure, radiating stria3 are 

 seen upon a little more than the anterior half of the shell; and upon the 

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

 cardinal border, there are small, somewhat distant, radiating striae, 

 easily seen by the unassisted eye. Upon these striae, both the distinct 

 and obscure, the lens shows numerous minute punctures, placed at 

 irregular intervals, which are the bases of minute, short, blunt spines, 

 or mark the places from which they have been removed. 



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

 millimeters ; greatest thickness, both valves together, sixteen millime- 

 ters. 



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

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

 that species, as figured and described by Stoliczska (Cretaceous Fauna 

 of Southern India, volume III, page 47, plate III, fig. 8; and plate XVI, 

 fig. 16), this specimen differs in being less globular, in having the 

 . umbones more elevated, and in the more distinct, radiating striae upon the 

 posterior half of each valve. 



This species belongs to an interesting group of shells, embraced within 

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

 generic name of Leiopistha, of which genus he regards the Cardium 

 elegantulum of Eoemer as the type. He has also divided the genus into 

 three sections or subgenera, under the names of Leiopistha proper, Cy- 

 mella, , and Psilomya (the latter yet in manuscript). The species here 

 described is regarded as a typical one of the last-named subgenus. The 

 completeness and precision with which that distinguished paleontolo- 

 gist has done the work upon this group of shells reflect great credit 

 upon the science, and it affords me pleasure to name this species in 

 his honor. 



Position and locality. -Strata of Cretaceous age, southeast of Paria, 

 Utah. 



