CRETACEOUS PEEIOD. 187 



tlie generic name of Leiojiistha, of which genus he regards the Cardium 

 eJcgantulum of Roemer as the type. He has also divided the genus into 

 thi'ee sections or subgenera, under the names of Leiopistha proper, Cymella 

 and Psilomya (the latter j^et in manuscript and awaiting publication). The 

 species here described is regarded as a typical one of the last-named sub- 

 genus. The completeness and precision with which that distinguished 

 paleontologist has done the work upon this group of shells reflect great 

 credit iipon 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. 



Subgenus CYMELLA Meek, 1864. 



Leiopistha (Cymella) undata Meek aud Hayden, sp. 



Plate XVIII, fig. 15 a. 



Pholadomya undata Meek and Hayden, 1856, Proc. Acad. 'S-a.t. Sci. Phila., 81. 

 Cymella undata Meek, 1864, Smithsonian Check- List Cret. and Jnrassic Fossils, 34. 



Shell small, subovate in marginal outline; valves moderately convex; 

 beaks somewhat prominent, incurving, and very slightly turned forward, 

 placed about two-fifths the length of the shell from the front; front margin 

 regularly, and basal margin more broadly, rounded; dorsal margin slojoing 

 a little downward to the front and behind, sloping gently to the posterior 

 margin ; the latter margin regularly, but a little more abmptly, rounded than 

 the front; concentric undulations comparatively strong and distinct, all 

 around, parallel with the free margins; radiating impressed lines of the 

 middle of the valves also distinct, but rapidly diminishing in distinctness 

 toward both ends; a narrow fold extends from beneath the beak, along the 

 postero-dorsal region, producing a narrow furrow along the lower side of 

 the fold, and a similar but less distinct one between it and the dorsal border, 

 both fmTOws being of about the same width as the fold. 



Length, thirteen millimeters; height, from beak to base, nearly ten 

 millimeters. 



This species is the type of the subgenus Cymella. 



Position and locality. — Strata of the Cretaceous period; Gallinas Creek, 

 New Mexico. 



