200 



THE EXTINCT BATRACHIA, REPTILIA 



Lor, as it has been accepted for the Mosasauroid included by Gibbes, by the next 

 writer, Leidy, it must be retained for it, according to the just rule usually followed. 

 There is, however, for us no evidence that the present genus possessed such a tooth, and 

 as the teeth of all the genera bear such a close mutual resemblance, I think it must be 

 left for future discovery to determine the application of the genus Holcodus. Name; from 



[Ran} -m\ oar. 



Dorsals transversely ovate, rounded; quadrate; bone broad as long, meatus larger; 

 humerus little contracted medially, with flat shaft; pterygoid teeth, eight. 



P. TYMPANITICUS. 



PLATECAKPUS TYM PANITICUS, Cope. 



Holcodus aeutidens, Leidy, Cretaceous Kept. N. Amer., p, lis. Tab. VII 4 7; VIII 1-2-7; XI 14; vix Gibbsii 

 Smithson. Contrib. 1851, II 7, Tab. I, 8-8, rel Leidyi, Loo. ('it. Tab. X-17. 



This species is of about the size of Mosasaurus depressus, Cope, under which head some of its characters are 

 pointed out. The single specimen representing it, whs found in the upper cretaceous of Mississippi, near Columbus, 

 by Dr. William Spillman. 



LIODON, Owen. 



Report (in British Fossil Reptiles. Proceedings Brit. Assoc Adv. Sci., 1841, p. 144. Odontography p. 261, Tab. 

 LXXII, fig. 1-2. 1 Maerosaurus, Owen, Journ. Geolog. Soc. London, 1840, 880. Cope, Proo. Bost. Boc. Nat. Hist., 

 isi;<). 864. ?Nectoportbeus, ('ope. Proc. Acad. Nat. Sci., 1868, 181. 



This genus was defined by Owen, in consequence of the more compressed and less 

 facetted character of the teeth in the type species, L. anceps, Ow. of England. This 

 character, though important, is shown by Leidy to be evanescent, as indicated by more 

 than one species of the United States. I find these species to differ materially from Mo- 

 sasaurus in the separation and linear form of the pterygoids. The characters are chiefly 

 derived from the L. proriger, Cope, and L. mitchillii, Dekay, as follows: 



The pterygoid bones without contact on the median line, but separated by a consider- 

 able interspace throughout their length, Pterygoid not entirely pleurodont. Chevron 

 hones free to the end of the vertebral column; dorsals with rudiment of the zygosphenal 

 articulation. 



These characters may he those of Macrosaurus, Owen, but the palatal characters of the 

 type of that genus are unknown; they are probably similar to those here given, as it ap- 

 proaches Clidastes in the form of the vertebral arches, and the pterygoids are separate in 

 the latter. The vertebra? known to belong to L. proriger present the compressed form of 

 those of Liodon la-vis. The vertebra of L. mitchillii are, however, unknown. The ptery- 

 goid tooth of a species like the last was named Les&kodus by Leidy. 1 have; not used this 

 name for the present genus, as the species and genus to which the type pertained are not 



