310 Miss Mary Johndon — A Jurassic Ganoid Fish. 



fish, about the specific determination of which there seems to be some 

 doubt. I therefore think the new specimen worthy of description, 

 and it is shown of half the natural size in the accompanying figure. 

 Nearly the whole of the fish is preserved in the fossil, but the head is 

 shown only in impression. Most of the scales are removed from the 

 near side, and the fins are fragmentary (see Plate XIII), 



The length of the head, with opercular apparatus, is approximately 

 equal to the maximum depth of the trunk, and contained six times 

 in the total length of the fish. As shown in impression, the frontal 

 region of the skull tapers rapidly to a point in front, where the 

 mesethmoid forms a small expansion, which is as broad as long. The 

 orbit is relatively large, and below it are traces of the bones of the 

 jaws. Among these may be distinguished a part of the arched 

 maxillary and impressions of some displaced fragments, bearing an 

 ornament of fine ridges. The preoperculum widens rapidly down- 

 wards to the angle, where the small expansion is shown to be smooth, 

 and below it are remains of broad branchiostegal rays, which are also 

 smooth. The operculum and suboperculum are shown only in 

 impression, but their outline is apparently complete. The operculum 

 is slightly narrower above than below, where its maximum width 

 exceeds two-thirds its depth. The suboperculum is about half as 

 deep as broad, with the posterior inferior angle rounded. 



The vertebral column is shown throughout the length of the fish at 

 places where the scales are removed. Centra are seen, but they must 

 have been only feebly ossified, perhaps in the form of rings, as they 

 are very much flattened. Fragments of the ribs are conspicuous on 

 the inner side of the scales, indicating that they also were feeUy 

 ossified, but were long and extended nearly to the ventral border. The 

 column ends at the beginning of the caudal fin, where it is slightly 

 upturned, with expanded haemal spines below its extremity. 



The form and proportions of the clavicular bones are not clearly 

 seen, but their external surface is shown to be ornamented with fine 

 vertical ridges. The rays of the paired fins are stout and flattened, 

 with a relatively long, unjointed base and very finely divided distal 

 end. In the remains of the pectoral fin the bases of the ten or twelve 

 rays are widely spread. The pelvic fins, which are inserted just 

 behind the anterior third of the trunk, are much smaller than the 

 pectorals and consist of seven or eight rays, of which the finely divided 

 distal ends are well preserved. The dorsal and anal fins are represented 

 by the merest traces, but the caudal fin is sufficiently well preserved 

 to show that it was forked and symmetrical. A few very slender 

 fulcra are seen on the lower border of the caudal fin. 



The scales, which completely cover the trunk, are thick and have 

 a highly polished smooth enamelled surface. Nearly all those on the 

 upper side of the fish have been lost, so that the less polished inner 

 surface of the under scales is mostly seen. The principal scales are 

 very finely serrated on their hinder margin. Their inner face is 

 strengthened by a vertical, wide, flattened rib, Avhich ends in a pointed 

 peg above, and is hollowed at the lower end by a triangular socket for 

 the peg of the next scale below. The excessively deepened flank- 

 scales rapidly decrease in depth in the hinder half of the tail, where 



