in North America of rare Extinct Vertebrates. 179 



sauroid reptiles the sternum has not yet been found so well 

 preserved as in Edestosaurus ; but there can be no reasonable 

 doubt of its presence. In Holosaurus there appears to have 

 been a partially ossified mesosternum" *. 



Thus to the restoration of Leiodon anceps in plate viii. 

 of vol. iv. 5th series of the Annals and Magazine Nat. Hist., 

 there may now be added not only the sternum, but also some 

 pairs of sternal ribs. 



A bone is described and figured as belonging to the hyoid 

 archf, of proportions rather Cetacean than Lacertian; it is still 

 more remote, in form, from any hyoid element of the Ophidian 

 type. 



Sclerotic plates " like those in Ichthyosaurus and a few 

 birds," forming a ring " composed of only a single row - of 

 plates, which, in position, overlapped each other "J, further 

 attest Saurian as against Ophidian affinities. I have accord- 

 ingly added such circle of ossicles to the orbit in the restora- 

 tion of Leiodon above cited. 



When I prepared the paper on the affinities of the Mosasau- 

 roids (1877), the homologue of the ectopterygoid in Ophidia was 

 not present in the specimens at my command, and had not 

 been noticed in any of Prof. Cope's examples. I could only 

 contrast with the palatal view of a Python's skull (ib. fig. 17) 

 the mutilated portion of the same surface of the skull in Mo- 

 sasaurus Hoffmanni (ib. fig. 16). The ectopterygoid has 

 now been recognized by Prof. Marsh in three American 

 genera of Mosasauroids ( Tylosaurus, Lestosaurus, and Edesto- 

 saurus)§. In Python the ectopterygoid is a long, slender, 

 narrow bone, having an oblique overlapping junction with the 

 otherwise free Rind end of the maxillary, and a similar but 

 more extended junction with the outer surface of the middle 

 expanded part of the pterygoid. In the Mosasauroids, as ex- 

 emplified by Tylosaurus, the ectopterygoid "is an L-shaped 

 bone, thin and somewhat twisted. One ramus unites by 

 suture with the corresponding process of the pterygoid ; and 

 the other extends forward, nearly at a right angle, to join the 

 maxillary "||. 



In the paper on the affinities of the Mosasauroids, it was 

 inferred from Prof. Cope's figures that the dentigerous palatal 

 bones, which he determined to be the true " palatines," were 

 the homologues of the bones described by Cuvier as the ptery- 

 goids in Mosasaurus*\. Prof. Marsh confirms this homology 

 and consequent affinity to the Iguanidas. Various specimens 



* Marsh, id supra, p. 83. f Ibid. p. 85, fig. 1. 



\ Ibid. figs. 2, 3, 4 (Lestosaurus simus, Marsh). 

 § Ibid. p. 86. || Ibid. 



f Owen, Quart. Journ. Geol. Soc. 1877, p. 698. 



