]901.] HBPTILES FEOM PATAGOISTA. 181 



The foremost pair at the symphysis is especially small, the tooth 

 almost completely preserved on the right being only about two- 

 thirds as high as the second tooth. Both these teeth are relatively 

 thick, being compressed to a sharp edge only at their concave 

 hinder border. The latter border seems to have been serrated 

 quite to the base ; but the anterior row of serrations scarcely 

 extends more than halfway down the crown, and is slightly 

 displaced from the median line towards the inner face of the tooth. 

 The following teeth, so far as preserved, are more nearly bilaterally 

 symmetrical, much compressed and indented near the base, with 

 the anterior serrations also extending at least halfway down the 

 crown. Except the third tooth on the left, and the fourth tooth 

 on the right side, all are fully extruded and nearly equal is size ; 

 and no traces of successional teeth are exposed. 



Simple compressed teeth, with more or less serrated edges, are 

 common to all the genera of carnivorous Dinosauria, and it is 

 difficult to discover diagnostic features solely in the jaws. Among 

 known jaws of this type, however, it does not seem necessary to 

 compare the new Patagonian specimen with any but those of 

 Megaloscmrus and Ceratosaurus — the former from Jurassic rocks in 

 England, the latter from a corresponding geological formation in 

 North America. If, as is commonly assumed, the number of 

 teeth in the prera axilla may be regarded as a generic character, 

 the fossil now described cannot be referred to Ceratosaurus, because 

 the type species of this genus exhibits only three premaxillary 

 teeth on each side\ In its possession of four premaxillary teeth, 

 on the other hand, the Patagonian jaw agrees with that oiMegalo- 

 saurus^; and it is dilRcult at first to perceive any essential 

 differences between these two fossils. The upper anterior extension 

 of the splenial bone has not hitherto been observed in Megalosaurus; 

 but there is a vacant hollow in the known specimens which may 

 have received it. There seem, however, to be important differences 

 in the inner wall of the mandibular tooth-sockets and in the 

 degree of development of successional teeth. Although the new 

 specimen is somewhat fractured, the inner wall of the dentary 

 completing the tooth-sockets appears to be continuous and as high 

 as the outer wall ; while in Megalosaurus, this inner wall consists 

 only of low lappets divided at the middle of each tooth by a large 

 clefts In the new specimen, moreover, very few successional 

 teeth are exposed ; whereas in Megalosaurus the apex of a successor 

 is conspicuous at the base of nearly every functional tooth. These 

 differences seem to necessitate 'the reference of the Patagonian 

 Dinosaur to a new genus, Genyodectes; and its type species, 

 represented by the jaw now described, may be named Genyodectes 

 serus *. Unfortunately, nothing is known of the jawe which bore 



^ O. C. Marsh, op. cit. p. 158, pi. viii. 



2 E. Owen, History of British Fossil Eeptiles, vol. iii. (1884), p. 169. 



^ E. Owen, op. cit. vol. i- p. 348, Dines, pis. xxxiii., xxxiv. 



* The so-called Loncosaurus argentinus (Ameghino, Anal. Soc. Cient. Argent. 

 Tol. xlvii. 1900, p. 61), a Megalosaurian from the Guarauitic Formation of the 

 Eio Sehuen, is not yet defined or sufficiently described for comparison. 



Peoc. Zool. Soc— 1901, Vol. I. No, XIII. 13 



