Reviews — Leidy's Cretaceous Reptiles. 433 



the figure of it would well pass. If really reptilian, we should place 

 it among the Mosasauri ; Professor Leidy suggests that it may belong 

 to either of his genera Discosaurus or Cimoliasaurus. But these are 

 Plesiosaurs, and with the Plesiosaurian tooth Polygonodon has nothing 

 in common. Fliogonodon is not figured ; the Professor suspects the 

 teeth, without assigning any reason, to have affinity with Mosasaurus. 

 To judge from his description of the circular crown, with numerous 

 plicse on the inside, they would appear to be mere Plesiosaurian. 

 Tomodon is thouglit likely by the author to be a tooth of Cimolia- 

 saurus or Discosaurus. It is short, flattened and broad, with finely 

 denticulated sharp borders in front and behind. We see no con- 

 clusive evidence that Tomodon is a reptile ; it certainly is as unlike 

 as could be to a Plesiosaur tooth, and is probably Mosasauroid. The 

 resemblance of the tooth called Piraiosaurus to Polyptychodon is 

 noticed, but it is really closer to some of the Plesiosaurs found at 

 the base of the English Oxford clay. Astrodon presents, as Professor 

 Leidy remarks, much resemblance to the teeth usually referred to 

 Hylceosaurus ; we should anticipate that they will prove to be the in- 

 cisor teeth of Eadrosaurus, just as the supposed teeth of Hylceosaurus 

 are probably the incisor teeth of Iguanodon. Trachodon only differs 

 from Hadrosaurus in the latter having the edges slightly serrated, 

 and is allowed by Professor Leidy to be probably referable to 

 Hadrosaurus. Discosaurus vetustus and Cimoliasaurus magnus are 

 Plesiosaurian vertebras of the common Cretaceous type, with flattened 

 articular surfaces, and for all that we can see to the contrary, should 

 both be referred to one species of Plesiosaurus ; Cimoliasaurus is 

 founded on the cervical and dorsal vertebrae ; Discosaurus on the 

 caudal vertebrae. The species is distinct from any yet published, 

 but apparently most nearly paralleled by one from the English 

 Portland Eock. 



Thoracosaiirus neocesariensis (de Kay) is the Crocodilus hasifissus 

 of Professor Owen. It is a true Crocodilian, and has its skull, teeth, 

 and some of the vertebrse figured, and in many details of the skull 

 presents considerable affinities with the Gavial. As the Professor 

 points out, the resemblance is close with the European Crocodilus 

 macrorliynclius figured in De Blainvilles' Osteographie. The formation 

 of a distinct genus could perhaps be sustained, but no evidence in 

 favour of such a step is adduced, and we are at a loss to understand 

 on what principle it is constituted, unless it be that every species 

 should be on a priori grounds the type of a genus. 



Bottosaurus Harlani is the Crocodilus basitruncatus of Owen. Pro- 

 fessor Agassiz made the genus. It appears to be more truly of the 

 alligator type than the previous species, so far as can be judged from 

 a few teeth, but not an atom of evidence is offered for the formation 

 of a distinct genus. 



In the final synopsis two species are added : Crocodilus tenehrosus 

 and C. ohscurus, founded on some specimens referred to in the body 

 of the memoir, but they are not placed m distinct genera. 



Polyptycliodon is quoted on the evidence of a single tooth, named 

 P. rugosus. But PolyptycJiodon teeth are quite undistinguishable from 



