AND KEPTILIA OF NORTH AMERICA. 



265 



The present genus resembles Lepterjjeton, HuxL, of the Irish coal-measures, in 

 the form of the cranium and in proportions of body, but that exhibits divided abdomi- 

 nal rods, or " oat-shaped scales " and the caudal vertebrae 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 Oestoceplialus, 

 but in that genus no pectoral shields have been observed. The vertebras in 0 remex 

 are of rather more elongate form than in the species of Ptyonius. Should, however, 

 the pectoral plates be found in O. remex, this genus must be united with that one. 

 Four species have been detected by the geological survey of Ohio. 

 I. Vertebrae shorter; fan-like processes of caudal vertebrae broad, equilatei-al. 

 a Abdominals rods coarser, not more than ten in .005 m. 



P Median pectoral plate broad, radiate ridged. P. marshii. 



aa. Abdominals rods hair-like, fifteen or more in .005 m. 



P Middle pectoral shield with radii from the centre, the principal forming a 



cross ; form wider. P. vinchellianus. 



P Middle pectoral with pits at the centre and few or no j-adii ; form narrow. 



P. PECTINATUS. 



P Middle j^ectoral shield narrow, closely reticulate medially, and radiate 

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



PTYONIUS MARSHII, Cope. 



C'olosteus marshii, Cope, Tran.sac. Amer. Philos. Soc, XIV., 18G9, p. 24; 0'eBtocej)lialus marshii, Cope, Proceed. 

 Amer. Pliilcs. Soc. 1871, p. 177. 



PTYONIUS YIN CHELLT ANUS, Coi^e. 



Oestocephalus vincJtellianus, Proceed. Amer. Philos. Soc. 1871, 177. 



Represented by the opposite halves of a single si^ecimen, -which includes only the cranium and anterior half of 

 the body. The fan-shaped neural spines commence but a short distance behind the line of the pectoral shields ; they 

 are low, and with a few coarse ridges ; the margin entire. The abdominal rods are delicate and hair-like. The 

 pectoral shield is an oval, with a few radiating crests which originate at the centre ; in the areas between these there 

 are a few scattered tubercles. The lateral shields are ridged near the margin. 



The cranium is lanceolate in form, and the bones of the superior walls are marked with a few raised points and 

 ridges. There is a thin bone which I have already alluded to as the squamosal or preoperculum, shaped like a right- 

 angled triangle separated from the outer posterior angle of the head, which exhibits a few similar.marks. 



M. 



Length of cranium, 0.020 

 Width " " , .008 



Length median j)ectoral plate, .0042 



This small species is about the size of P. pecAinatiat, and should be especially compared with it. In specimens 

 A. P. R. \OJj. x\\ 3o. 



