490 GEOLOGICAL SURVEY OF THE TEREIT0RIE3. 



erly fall into that geuus, either as a snbgenus or otherwise, then the 

 uame Pteria will have to be substituted for Avicula in connection with 

 these species. 



Locality and position,— CoalviWe, Utah : from white sandstone, 250 

 feet above the lower heavy bed of coal, mined at that place. 



Avicula (Pseudoptera) ehytophora, Meek. 



Shell, as determined from a left valve, but slightly oblique, rhorabic- 

 snboblong, and nearly twice as high as wide in adult examples, but 

 proportionally broader and subtrigonal in young si)ecimens ; moder- 

 ately convex, the greatest convexity being toward the anterior side, 

 along the umbonal slope, which appears to be angular, thence cuneate 

 posteriorly, and more. or less deliected inward anteriorly; hinge line 

 very nearly equaling the greatest anteroposterior diameter, and rang- 

 ing at an angle of about 70° to the umbonal axis ; posterior margin 

 nearly straight, or a little convex in outline along the middle, where it 

 ranges at an angle of about lOU^ to the hinge margin, but curving a 

 little forward above, so as to connect with the latter at a somewhat 

 more obtuse angle, while below it curves gracefully downward and for- 

 ward into the narrowly rounded or somewhat angular base ; anterior 

 margin a little sinuous in outline in the middle, with a general direction 

 nearly parallel to that of the umbonal slope, but compressed nearly 

 rectangular, and projecting a little beyond the beak above, the project- 

 ing part not having tbe character of an ear or distinct lobe, though 

 defined by a shallow depression extending from the beak obliquely 

 downward and backward to the slightly sinuous central region; beak 

 very nearly terminal, moderately oblique, and rather compressed. Sur- 

 face with more or less distinct lines of growth, and near the hinge mar- 

 gin well-defined, regular, vertical ridges or wrinkles, that seem not to 

 be exactly parallel to the lines of growth. Eight valve not certainly 

 known. 



Height of right valve, 3.20 inches; antero-posterior diameter along 

 hinge line, 1.90 inches; height about half way down parallel to hinge, 

 two inches ; convexity, 0.70 inch. 



This species will be readily distinguished from the last, not only by 

 its much larger size and less oblique and broader form, but also by the 

 strong vertical wrinkles along its hinge margin. It likewise seems to 

 be entirely without any traces of the radiating costa? seen on the ante- 

 rior side of that species, and has its posterior margin -much more nearly 

 vertical above, and slightly convex in outline, instead ot a little sinuous 

 there. Its umbonal slope, in the only left valve seen, seems to be 

 decidedly angular, though this may be partly, if not entirely, due to an 

 accidental fracture and bending of the valve along that line. It looks, 

 however, like a natural angle, with some little uotles or projecting points 

 along its crest. In general form it presents much the outline of some 

 of the large Myalinas of the western coal-measures, such as 2L ampla 

 and M.sitbquadvata, but it differs not onlj' in its angular umbonal slope, 

 less curved beaks and wrinkled dorsal margin, but in having its ante- 

 rior margin flattened and a little extended beyond the beak in front, 

 instead of being concave in outline there, thus not leaving the beak 

 quite terminal, as we see in Mi/alina. 



I am not quite sure that I have seen the right valve of this shell, 

 though one of tlie same general outline, and of corresi)onthng size, that 

 ■was observed in a large mass of rock at the locality, was believed to 



