A treatise on zoology . 127), whence theirname Sting-Rays. These spines are placed behind the fins, not in frontas in other Selachians. Xyphotryrjon, Cope ; Eocene, N. America. Urolophus, M. and H., andTrygon, Ad. ; warm seas, and to Eocene, Italy. Uroyymnus, M. and H.;Indian seas. Pterojjlatea, M. and H. (Figs. 92, 127); temperate andtropical seas. Tryyonoptera, M. and H. ; Australian. RAJIFORMES 165 Family Ptychodontidak. A group of Cretaceous tish of wliiclilittle is known except tlie teeth. The dentition somewhat resemblesthat of the Myliobatidae, there being longitudinal rows of teeth, of
A treatise on zoology . 127), whence theirname Sting-Rays. These spines are placed behind the fins, not in frontas in other Selachians. Xyphotryrjon, Cope ; Eocene, N. America. Urolophus, M. and H., andTrygon, Ad. ; warm seas, and to Eocene, Italy. Uroyymnus, M. and H.;Indian seas. Pterojjlatea, M. and H. (Figs. 92, 127); temperate andtropical seas. Tryyonoptera, M. and H. ; Australian. RAJIFORMES 165 Family Ptychodontidak. A group of Cretaceous tish of wliiclilittle is known except tlie teeth. The dentition somewhat resemblesthat of the Myliobatidae, there being longitudinal rows of teeth, of whichthe middle row is the largest (Fig. 128). The large teeth are nearlysquare in shape, and composed of thick vasodentine, with a ridgedsurface (Owen [311a], A. S. Woodward [505«]). In the shape of the jawthey approach the Trygonidae. Ptychodas, Ag. ; Cretaceous, Europe and N. America. Family Myliobatidae. As in the last family, the pectoral finsgenerally meet in front of the skull ; but their anterior, preorbital. Lower jaw of Ptyclwdus decurrens, Ag. ; Cretaceous, Sussex. (After A. S. Woodward,from Brit. Miis. Guide.) portions become separated off from the rest of the fin, so as to form aspecial anterior cephalic fin, median in the Myliobatinae (Fig. 129, B).In the Dicerobatinae this fin is developed as paired horn-like projections(Fig. 129, C). The head, with its large protruding eyes, projects dorsally above thelevel of the disk. The skeleton resembles that of the Trygonidae ; butthe ceratohyal is attached to the first branchial arch. The skin is smooth,excepting for a strong serrated spine which generally lies behind a singlesmall dorsal fin, near the base of the elongated whip-like tail (Fig. 129). Sub-Family 1. Myliobatinae. With a median cephalic fin,quite continuous with the pectorals in Promyliobatis. The teeth form aflat grinding pavement (Fig. 130), of which the central Iow becomes 166 SELACHII greatly enlarged and transversely elongated. They form a mosaic ofcl
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