WHITE] PALEONTOLOGY — CRETACEOUS FOSSILS. 283 



and more or less deflected inward anteriorly; liinge-line very nearly 

 equaling' the greatest autero-postorior diameter, and ranging at an tingle 

 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 100° 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 giacefidly downward into the narrowly rounded or some- 

 what 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 projecting part not having the character of an ear 

 or distinct lobe, though defined by a shallow depression extentling from 

 the beak obliquely downward and backward to the si ightly sinuous cen- 

 tral region; beak very nearly terminal, moderately oblique, and rather 

 compressed. Surface with more or less distinct lines of growth, and near 

 the hinge-margin Avell 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, 2 

 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 costse seen on the anterior 

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

 tical above, and shghtly convex in outline, instead of a little sinuous 

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

 cidedly angular, though this may be partially 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 nodes 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 31. ampla and 

 M. siibquadrata, but it diflters not only in its angidar umbonal slope, less 

 curved beaks, and wrinkled dorsal margin, but in having its anterior 

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

 of being concave there, thus not leaving the beak quite terminal, as we 

 see in Myalina. 



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

 though one of the same general outline and of corresponding size that was 

 observed in a large mass of rock at that locality was believed to belong 

 to this species.* It was nearly flat and smooth, excepting fine lines of 

 growth, and, if I mistake not, had a tolerably deep, weU-defined byssal 

 notch. If it really belonged to this shell, the species can hardly go 

 properly, as already stated, into the group Fseudoptera, the type of 

 which has no traces of a byssal sinus in either valve." 



As before stated, I regard both these forms as belonging to one and the 

 same species. Figures 2 a and b on plate x are drawn from Mr. Meek's 

 types of his Avictila {Pseudoptura) propletira, and fig. 3 a from a gutta- 

 percha cast of his type (which is a natural mold in sandstone), and only 

 example of his Avicula {Psendopfcra) rliytopliora. The latter is doubtless 

 an old shell, while the others are not quite adult. The vertical wrinkles 

 seen near its hinge-margin are faintly indicated upon some of the other 



*'*The specimen was broken into fragments in trying to detach it from the mass of 

 rock." 



