492 GEOLOGICAL SURVEY OF THE TERRITORIES. 



Iviiul and restore both to Avicida, wliicli I am certainly not inclined to 

 do, tliongli I regard Oxytoma as a subgenus under Avicula. 



I use tlie name Aricula here, as elsewhere, subject to the change that 

 it is probable the rules of nomenclature will demand in the restoration 

 of the older name Pteria, which would require the name of our species 

 to be written Ptcria gastrodes, if it falls into that group. 



Locality and jjosition. — Cretaceous sandstones of Coalville, Utah. 



MoDiOLA (Brachydontes) multilinigera, Meek. 



ModcoJa Pedcrnalis, Meek, 1870; Haj^den's Geol. Report, Wyoming, »fcc., List Cretaceous 



fossils, page 297, (not Roeiner.) 



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

 strongly convex along the umbonal sloi)es, thence cuneate posteriorly, 

 and abruptly curved inward below the middle in front ; posterior margin 

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

 ward to the auteiior basal extremity, which is very narrowly and ab- 

 ruptly rounded ; anterior margin ranging obliquely backward and down- 

 ward 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 protube- 

 rance; hinge margin nearly or quite straight, ranging at an angle of 

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

 prominent part of the basal outline, and equaling about half the greatest 

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

 very oblicpie, and scarcely rising above the liinge margin; umbonal 

 slopes prominent and more or less strongly arcuate. Surface ornamented 

 by fine 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 lai'gest being near the dorsal 

 side, where they bifurcate so as to become very fine, and curve more or 

 less upward before reaching the cardinal margin. 



Greatest length, measuring from the beaks 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. 

 Hay den from near Coalville, Utah, I was led to think it probably the 

 form described by Dr. Iloemer from Texas, under the name Modiola 

 Pcdcrnalis, to which 1 referred it provisionally, in making out the list of 

 Cretaceous fossils for Dr. Hayden's report of 1870. Further comparisons 

 of better specimens collected during the past summer at the same locality, 

 however, have satisfied me that it presents well-marked and constant 

 difi'ereuces from the Texas shell. In the first place, it is distinctly more 

 arcuate, so much so, that when placed with its hinge line in a horizontal 

 position, the outline of its posterior margin, instead of forming an 

 oblique backward descending curve, ranges nearly vertically. Again, 

 the most prominent part of its posterior basal margin 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 beaks and in front of the 

 umbonal ridge, also differs in being proportionally much smaller than 

 in Dr. lioemer's species, in whicli it forms about one-third of the entire 

 valve, as seen in a side view^; while in our shell it scarcely forms more 

 than one-sixth. Of course the specimens are more or less variable in 



