Type: Specimen No. 8546 G, American Museum of Natural History. No. 8614 G, same museum, is associated with the type specimen. Horizon and locality: Linton, Ohio, Coal Measures. Two well-preserved individuals display peculiarities which indicate specific distinctness from the previously known species of _Ptyonius_. The abdominal rods are of the coarse type of those of _P. marshii_. The caudal fans are well developed, and not so wide as in _P. pectinatus_. The interclavicle is a discoid body of different form from that of _P. marshii_ and I can not detect the clavicles. The sculpture consists of strong ridges, which radiate from the center to near the border. Immediately in front of this interclavicle is the head, which has a different form from that of the other known species. The interorbital width is two-thirds the long diameter of the orbit. The structure of the skull can not be made out. A slender, elongate hind limb is present in the second specimen, and a humerus is well preserved in the type. Measurements of the Type of Ptyonius nummifer Cope. (No. 8546 G, American Museum of Natural History. No. 8614 G is associated in the same species.) mm. Length to beginning of caudal fans 65 Length of head 15 Length from muzzle to orbits 6 Length of interclavicle 7 Width of interclavicle 8 Width of abdominal armature 8 Length of a caudal fan 2.5 Length of femur 5 Proximal width of femur 1.5 =Ptyonius serrula Cope.= Cope, Proc. Am. Phil. Soc., 1871, p. 177. Cope, Geol. Surv. Ohio, II, pt. II, p. 379, pl. xxviii, fig. 5; pl. xxx, fig. 1, 1875. Type: Specimen No. 8615 G, American Museum of Natural History. Horizon and locality: Linton, Ohio, Coal Measures. The specimens of this species indicate that the form was only about half the size of _Ptyonius pectinatus_. The interclavicle is narrower and more reticulately sculptured. The tail is relatively longer. Abdominal rods hair-like. Ribs distinct. Small limbs are present in one specimen. Measurements of Cope's Type of Ptyonius serrula. mm. Length of specimen 95 Length of portion of skull preserved 3 Length of interclavicle 4.5 Width of interclavicle 2 Length of clavicle 4 Width of clavicle 1.5 Length of vertebra 1 Width of vertebra from tip of neural spine to tip of hæmal spine 4 Another specimen (456 G, American Museum of Natural History) shows some of the same characters. There is not the slightest basis for the support of this species, so far as I can observe. The ones mentioned by Cope are insufficient. It is in all probability a mutant or variety of _Ptyonius pectinatus_.