74 SYSTEMATIC PALEONTOLOGY 



Myliobatis gigas Leidj-, 1877, Jour. Acad. Nat. Sci. Phila., 2nd ser., voL viii, p. 241, 



pi. xxxiii, fig. 4. 

 Myliobatis vicomicanus Leidy, 1877, Jour. Acad. Nat. Sci. Phila., 2nd ser., vol. viii 



p. 242, pi. xxxiii, fig. 5. 



Description. — Dentition large but comparatively thin, the smooth 

 coronal contour nearly flat in the lower jaw and but slightly arched 

 from side to side in the upper. Longitudinal superficial striae well- 

 marked, regularly deflected outwards on passing posteriorly. Median 

 teeth in the adult al)out nine times broader than long, more arched 

 at the sides than in the middle; lateral teeth longer than broad. Trans- 

 verse sutures of median teeth slightly recurved posteriorly along the 

 sides, and to a lesser extent (in the lower dentition) also in the middle. 



This species is remarkable for the great tenuity of the tesselated 

 pavement in proportion to its size, just as M. pacliyodon is remarkable 

 for its excessive thickness. These differences are best understood by 

 a comparison of the cross-sections given on Plate XXVIII, Fig. 3b, and 

 Plate XXIX, Figs, lb, of this volume, with Plate XIll, Fig. la, of the 

 Eocene volume. The lower dental pavement is relatively narrower than 

 the upper, and its median teeth are shorter. In the type-specimen of 

 the so-called "if. vicomicanus/' shown in Plate XXIX, Fig. 1, the 

 median teeth are fully nine times as broad as they are long. Cope's 

 types of this species have already been figured by Leidy, although for 

 some unexplained reason certain fragments belonging to the left-hand 

 side of the upper dentition in front were omitted by the artist. 



The transverse sutures, especially those of the lower dental pavement 

 and the longitudinal superficial striae, are curved similarly to those of 

 M. magister from the Eocene Phosphate Beds of South Carolina ; but the 

 median teeth are shorter, fiatter and much thinner than in the Eocene 

 species. The lower dental pavement exhibits a shallow longitudinal 

 depression along the median line, in which the transverse sutures are 

 gently curved posteriorly. In this respect the lower dentition resembles, 

 that of M. magister Leidy, M. dixoni Agassiz, and some other species, 

 while it is exactly opposite to the condition presented in the upper 

 dentition of M. fastigiatus Leidy. 



The total length of the series of eleven median teeth in the lower 

 dental pavement shown in Plate XXIX, Fig. 1, (type of M. vicomicanus) 



