6TANT0N.] ANATINID^E. 119 



LlOPISTHA (PSILOMYA) CONCENTRICA n. Sp. 



PL xxvi, Figs. 8-10. 



Shell of medium size, very thin, transversely ovate in outline, mod- 

 erately convex $ anterior and basal margins forming a continuous but 

 unequally convex curve ; posterior end subangular and compressed; 

 dorsal margin behind the beak, straight, slightly descending, abruptly 

 deflected so as to form a distinct false area which bears fine transverse 

 striae and is separated from the body of the shell by a narrow furrow 

 running from the beak to the angular posterior end; beaks prominent, 

 ventricose, incurved, and but slightly oblique, situated a little in ad- 

 vance of the middle. Surface marked by lines of growth and regular 

 concentric undulations that are about l mm apart on the body of the 

 shell, but are more closely arranged and more prominent near the 

 beaks. 



Length of one of the largest specimens, 27 mm ; height, 19 mm ; convex- 

 ity of a single valve, 8 mm . 



This species has very nearly the size and shape of L. meelci. It is 

 slightly more elongate, however, and its surface ornamentation is 

 entirely different. The Survey collections contain more than a dozen 

 of each of these species, and the differences seem to be constant. The 

 Utah species, L. meeki, never has the concentric undulations, and 

 always shows more or less radiating striae, while in L. concentrica the 

 reverse of this is true. 



Locality and position. — In the Pugnellus sandstone on Williams creek 

 and in Poison canyon, Huerfano park, Colorado, and at about the same 

 horizon on the Arkansas river, 20 miles above Pueblo, Colo. 



Note.— Since the above description was written about 25 additional specimens of Liopistha meeki 

 have been collected in southern Utah, and among them is one marked with distinct radiating striae 

 and with concentric undulations almost as strong as in L. concentrica. Possibly the latter should be 

 regarded as only an extreme variety of L. meeki, though the two forms are easily distinguishable. 



Liopistha (Psilomya) elongata n. sp. 

 PI. xxvi, Figs. 11 and 12. 



Shell rather large for this genus, inequilateral, transversely elongate, 

 moderately convex in the umbonal region, and compressed posteriorly 5 

 dorsal margin behind the beak, straight, and inflected so as to form a 

 narrow false area; anterior margin forming a regular curve from the 

 beak to the ventral margin, which is but slightly curved and almost 

 parallel with the dorsal margin ; posterior end subtruncate above and 

 narrowly rounded into the base below; beaks prominent, ventricose, 

 incurved, and approximate, situated about half-way between the middle 

 and the anterior end. Surface marked by lines of growth and obscure 

 concentric undulations that are most prominent on the umbonal region 

 and are as distinct on the casts as on the surface. 



Length, 37 mm ; height, 27 mm ; convexity of a single valve, 9 mm . 



