42 PEOCEEDINGS OF THE ACADEMY OF 



muscular and simple pallial scars, together with the remains of its external 

 ligament, we can have little doubt in regard to its being a Schizodus. In size 

 and general outline, its internal cast closely resembles Amphiccelia Leidyi, de- 

 scribed by Prof. Hall from the Upper Silurian Limestone at Bridgeport, near 

 Chicago. It has, however, obviously no near relations to that shell. For a 

 Schizodus, its beaks are unusually depressed and oblique. We know of no 

 other species resembling it in general form. 



Compared with Schizodus occidentalis (= Cypricardia ? occidentalis, Swallow), 

 which seems to be a true Schizodus, and agrees with our shell in size, it is at 

 once distinguished by its much more depressed beaks, less convex valves, and 

 broadly round posterior outline. Prof. Swallow's species also occurs in our 

 Coal Measures, but we have yet only seen it from a higher part of the series, 

 at Lasalle. 



Locality and position. Seaville, Fulton County, Illinois, Coal No. 1, of Coal 

 Measures. 



Schizodus (Prisconaia) perelegans, M. and W. 



Shell attaining a moderately large size, ovate-subcordate in general form, 

 very gibbous, apparently closed all around, rather thin. Anterior and poste- 

 rior views presenting a neat cordate outline. Pallial margin semiovate, the 

 most prominent part being in front of the middle ; anterior side very short, 

 and rounding up abruptly from below ; posterior side narrowly rounded, or 

 subangular; hinge line apparently rather short. Beaks very gibbous, promi- 

 nent, pointed, and strongly incurved, so as to bring their points nearly or quite 

 in contact; located near the anterior margin; lunule excavated, but not dis- 

 tinctly denned. Surface appearing to the unassisted eye almost perfectly 

 smooth, excepting very regular, raised, concentric stria?, or small costre, on 

 the anterior part of the valves, with obscure traces of smaller stria? crossing 

 the same. Under the highest power, however, that can be well used as a hand 

 magnifier, the whole surface is seen to be covered by an extremely minute, 

 perfectly regular, crowded sculpturing, as if made by crowded, microscopic 

 cross lines, entirely invisible even under a good common pocket magnifier. 



Height of a small specimen, 0-90 inch; convexity of same, 0-80 inch; 

 length, about 1*18 inches. Other imperfect specimens show that the species 

 sometimes attained three or four times the size of that from which the above 

 measurements were taken. 



The extremely minute ornamentation mentioned on this shell may be prop- 

 erly considered microscopic, being entirely invisible under a good common 

 pocket lens, by the aid of which it shows apparently an almost polished 

 surface. When examined in a strong light, however, with a sufficiently high 

 magnifier, the finer markings are seen to present a delicacy, fineness, regu- 

 larity, and beauty that art could scarcely attain, even by the aid of the most 

 accurate mathematical ruling machine. In some specimens this delicate 

 marking has been, in places, partly removed, as if by the abrasion of an ex- 

 tremely thin external layer, or epidermis, to which it appears to be, to some 

 extent, confined. When this layer has been in this way removed, however, 

 traces of the same marking can be seen on the surface of the layer beneath, 

 by the aid of a sufficiently strong magnifier. 



The stronger concentric, raised stria?, or small costas, seen on the anterior 

 part of the valves, do not run exactly parallel to the very obscure marks of 

 growth, but cross them obliquely. On following these little costae back 

 toward the gibbous part of the valves, they all seem to become suddenly ob- 

 solete ; but on examining them with a common pocket lens, they are seen to 

 be continued over the convex part of the valves, as minute, impressed hair 

 lines, very regularly disposed, so as to leave comparatively rather wide spaces 

 between. Here, too, they are seen not to be exactly parallel to the faint 

 marks of growth, and on following them back they are observed to terminate 

 suddenly at a similar impressed hair-line, descending with a slight curve from 



[April, 



