86 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



Subgenus IDONEARCA. 

 Cucullsea (I don cure a) Shumardi, M. & H. 



Plate 28, figs. 15 a-ij ; and pi. 29, lig. 4. 



Area (Cucullwa) Shumardi, Meek and Hayden (Apr., 185C), Proceed. Acad. Nat. Sci. Philad., VIII, 86. 

 Cueullma Shumardi, M. & H. (Nov., 185G), ib., 285. 

 Cucullwa fibrosa, M. & H. (Oct., 18G0), ib., xii, 428. 



Compare Ctu-ullasa fibrosa, Sowerby (1818), Mineral Conch., Ill, 9. 



Shell transversely rhombic-oval, very slightly inequivalve, entirely closed, 

 rather thin; anterior margin rounded, sometimes intersecting the hinge above 

 so as to form an obtuse angle, rounding more or less obliquely into the base 

 below; basal margin semi-ovate in outline, the most prominent part being 

 behind the middle, not crenulate within ; posterior side obliquely truncated 

 above, and narrowly rounded below; hinge less than three-fourths the entire 

 length of the shell; cardinal area rather narrow, marked by very fine longi- 

 tudinal striae, and about four strong, oblique, divaricating grooves; beaks gib- 

 bous, moderately elevated, incurved nearly at right angles to the cardinal 

 line, and located slightly in advance of the middle. Surface marked by con- 

 centric striae, which are crossed on young shells by small, obscure, radiating 

 costae, which become nearly or quite obsolete on adult specimens. 



Length of a large specimen, 1.67 inches; height from base to top of 

 beaks, 1.40 inches; convexity, 1.24 inches. 



This species varies considerably in form, and to some extent in its sur- 

 face-markings, at different stages of its growth; young individuals being gen- 

 erally shorter, less oblique, and more distinctly truncated behind, as well as 

 more strongly marked by the radiating lines or costse. As the shell advanced 

 in its growth, the posterior ventral region became proportionally more extended, 

 so as to give the valves a. more oblique outline; while the radiating costae 

 gradually became obsolete, and the concentric markings stronger. Although 

 these differences arc in most cases due to age, such is apparently not always 

 the case, as we sometimes, though less frequently, see small individuals with- 

 out any traces of the radiating costae. There arc, however, so many grada- 

 tions between these extremes that they cannot be considered distinct species. 



On the outside of some specimens, a single raised line extends from the 

 back part of the beak of the right valve obliquely backward to the middle of 

 the posterior margin, following the direction of ll 



