JURASSIC SPECIES. 135 



incurved, and very nearly central ; posterior umbonal slopes more or less 

 angular, while the posterior dorsal region above these oblique umbonal 

 ridges is compressed. Surface neatly ornamented with small, regular, 

 radiating lines, or raised striae, which are rather widely separated and larger 

 on the anterior part of the valves, where they curve gracefully forward, and 

 alternate with a smaller intermediate series, none of which are continued to 

 the beaks ; posteriorly they all become much smaller, and closely crowded ; 

 Grossing all of these, there are, on all parts of the valves, numerous very 

 small regular and crowded concentric lines. 



Length, 0.57 inch; height, 0.35 inch ; convexity, 0.30 inch. 



This species seems to be closely allied to, and may even be identical with, 

 some of the forms that have been referred to Cucullcea concinna, Phillips, by 

 different authors. It certainly differs, however, very materially from the 

 type of C. concinna, as originally illustrated by Phillips, in being much less 

 depressed, proportionally shorter, decidedly less angular along the poste- 

 rior umbonal slopes and at the posterior basal extremity. Phillips' figure 

 also shows obscure, coarse, radiating costse, on the posterior surface of the 

 valves- above the umbonal angle, not seen on our shell, which likewise 

 differs in having more prominent and more gibbous umbones, with a more 

 rounded basal outline. It is more nearly like the form figured under Phillips' 

 name by Goldfuss (Petref. Germ., plate cxxiii, fig. 6 a, h), and by Quenstedt 

 (Der Jura, tab. 67, fig. 16); but it is less angular at the extremities, and has 

 more gibbous umbones. It may also be compared with a shell figured from 

 the Jura by Richard Andree, in the Zeitschrift der Deutschen Geologischen 

 Gesellschaft, XII, plate xiv, fig. 7, under d'Orbigny's name Area subconcinna, 

 which, however, is less depressed, much more rounded in outline, and has 

 smaller and lower umbones than our shell. 



Locality and posiUon. — Weber Canon, Wasatch Range, Utah ; from a 

 Hmestone apparently of Jurassic age. 



