PALiEONTOLOGY OF OHIO. 



of the former to an operculum in these the most fish-like of the Batra- 

 chia. The teeth are numerous, small, and some of them apparently 

 simple ; others appear to be grooved. In a cranium (No. 140), perhaps 

 of P. pectinatus, they extend to the tip of the slender jaws, are rather 

 stout, acute, and evidently marked with a few strong grooves on the 

 shank. The form of the head is a curious miniature of Ichthyosaurus. 



Remains of limbs have only been observed in the position of the pos- 

 terior pair, and that in several individuals. 



The present genus resembles Lepterpeton, HuxL, of the Irish Coal 

 Measures, in the form of the cranium and in proportions of body, but that 

 exhibits divided abdominal rods, or " oat-shaped scales," and the caudal 

 vertebrte have not the fan-like processes. In the last point they agree 

 with Urocordylus, Huxl., but this genus is not represented as possessing 

 ribs, and the abdominal rods are also divided, forming the " oat-shaped 

 scales." I formerly referred the species of Ptyonius to Oestocephalus, but 

 in that genus no pectoral shields have been observed. The vertebix in 



0. remex are of rather more elongate form than in the species of Ptyonius. 

 Should, however, the pectoral plates be found in 0. remex, this genus 

 must be united with that one. 



Four species have been detected by the Geological Survey : 



1. Vertebrae shorter; fan-like processes of caudal vertebrje broad, 

 equilateral. 



X Abdominal rods coarser, not more than ten in .005 m. 



Median pectoral shield discoid, radiate ridged; muzzle short P. nummifer. 



Median pectoral shield oval, pitted and ridged P. Marshii. 



XX Abdominal rods hair-like, fifteen or more in .005 m. 



Middle pectoral shield with radii from the center, the principal form- 

 ing a cross; form wider P.VinclielUarms. 



Middle pectoral with pits at the center and few or no radii ; form 

 narrow P. pectinatus. 



Middle pectoral shield narrow, closely reticulate medially, and radiate 

 towards the circumference; size half that of the last P. serrula. 



Ptyonius nummifer, Cope. 



Two well-preserved individuals display peculiarities which indicate 

 specific distinctness from previously known species of Ptyonius. The ab- 

 dominal rods are of the coarse type of those of the P. Marshii. The 

 caudal fans are well developed, and not so wide as in P. pectinatus. The 

 pectoral scuta form a discoid body of a difterent form from that of P. 

 Marshii, and I can not detect the lateral scuta at all. The sculpture con- 

 sists of strong ridges, which radiate from the center to near the border. 



