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Show 1901.] REPTILES FROM PATAGONIA. 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 difficultto 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 Megalosaurus 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 premaxilla may be regarded as a generic character, the fossil n o w described cannot be referred to Ceratosaurus, because the type species of this genus exhibits only three premaxillary teeth on each side1. In its possession of four premaxillary teeth, on the other hand, the Patagonian jaw agrees with that of Megalosaurus 2; and it is difficult 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 cleft3. 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 serus4. Unfortunately, nothing is known of the jawo which bore 1 O. C. Marsh, op. cit. p. 158, pi. viii. 2 R. Owen, History of British Fossil Reptiles, vol. iii. (1884), p. 169. 3 R. Owen, op. cit. vol. i- p. 348, Dinos. pis. xxxiii., xxxiv. 4 The so-called Loncosaurus argentinus (Ameghino, Anal. Soc. Cient. Argent. vol. xlvii. 1900, p. 61), a Megalosaurian from the Guaranitic Formation of the Rio Sehuen, is not yet defined or sufficiently described for comparison. PROC. ZOOL. Soc-1901, VOL, I. No. XIII. 13 |