STANTON.] MYTILIDiE. 87 



Compare Modiola tenuisculpta Whiteaves, 1889, Cont. to Can. Paleontology, vol. i, p. 

 188, PI. 26, Figs. 2 and 2a. 



Original description: 



" Shell rather above medium size, obliquely arcuate-subovate; valves 

 strongly convex along the umbonal slopes, thence cuneate posteriorly, 

 and abruptly curved inward below the middle in front; posterior mar- 

 gin forming a broad, regular, convex curve, from the end of the hinge 

 downward to the anterior basal extremity, which is very narrowly and 

 abruptly rounded ; anterior margin ranging obliquely backward and 

 downward to the narrow basal extremity, and strongly sinuous along 

 the middle, above which it projects more or less beyond the umbonal 

 ridge, so as to form a moderately prominent, somewhat compressed 

 protuberance; hinge margin nearly or quite straight, running at an 

 angle of 50° to G0° above an imaginary line drawn from the beaks to 

 the most prominent parts of the basal outline, and equaling about half 

 the greatest oblique length of the valves ; beaks nearly terminal, rather 

 compressed, very oblique, and scarcely rising above the hinge margin ; 

 umbonal slopes prominent and more or less strongly arcuate. Surface 

 ornamented by line lines of growth, crossed by regular radiating lines 

 that are very fine and crowded on the anterior part of the valves, but 

 become coarser above and behind the umbonal ridge, the largest being 

 near the dorsal side, where they bifurcate so as to become very tine, 

 and curve more or less upward before reaching the cardinal margin. 



" Greatest length, measuring obliquely from the beaks to the most 

 prominent part of the basal margin of a large specimen, 1.90 inches; 

 greatest breadth at right angles to the same, 1 inch; convexity, 0.76 

 inch. 



"On first examining some imperfect casts of this shell, brought by 

 Dr. Hayden from near Coalville, Utah, 1 was led to think it probably 

 the form described by Dr. Eoemer from Texas, under the name Modi- 

 ola pedernalis, to which I referred it provisionally, in making out the list 

 of Cretaceous fossils for Dr. Hay den's report of 1870. Further com- 

 parisons of better specimens collected during the past summer at the 

 same locality, however, have satisfied me that it presents well-marked 

 and constant differences from the Texas shell. In the first place, it is 

 distinctly more arcuate v so much so, that when placed with its hinge 

 line in a horizontal position, the outline of its posterior margin, in- 

 stead of forming an oblique backward descending curve, ranges nearly 

 vertically. Again, the most prominent part of its posterior basal mar- 

 gin is very narrowly rounded, instead of forming a regular curve. Its 

 umbonal ridges are likewise more prominent, more arched, and extend 

 down to the narrowly rounded posterior basal extremity. The lobe-like 

 projection of the upper part of its anterior margin, under the beak and 

 in front of the umbonal ridge, also differs in being proportionally much 

 smaller than Dr. Roemer's species, in which it forms about one-third of 

 the entire valve, as seen in a side view, while in our shell it scarcely 



