752 PK0E. OWEN ON THE AFFINITIES OF THE MOSASATTRIDJK. 



vertebrae, in which he states that I " will find the zygosphene di- 

 stinctly represented " *. If that part had been indicated by any 

 letter or mark, I should gladly have recognized it ; for the alleged 

 structure is an interesting addition to the osteology of the Mosa- 

 sauroids. 



Accepting the testimony of these figures, according to the author's 

 determination of the parts of the fractured vertebras, the Mosasaurian 

 skeleton offers, in the genus Clidastes, another ground of comparison 

 with the Ophidian, on the one hand, and the Lacertian on the other. 

 Certain members of the latter order, forming the family of the 

 Iguanidse, show, in the trunk-vertebrae, the same super-additions 

 which characterize those of the Ophidia. But there are modifica- 

 tions of the zygosphene and zygantrum which, with minor differences, 

 distinguish the Iguanian from the Ophidian vertebrae. 



I venture to repeat, therefore, the suggestion in the paper above 

 cited, p. 709, of a desire to possess an entire vertebra, or a cast of 

 such, of a Mosasaurian showing the zygosphene and zygantrum, 

 in order to compare it with an Ophidian vertebra (fig. 24, p. 709, 

 Quart. Journ. Geol. Soc, Nov. 1877") and an Iguanian vertebra (fig. 

 22, he. cit.). 



The determination of the affinities of an extinct reptile, or other 

 organism, affords a firm step to higher generalizations ; and our 

 obligations are truly great to Professors Marsh and Cope for the rich 

 additions which their zealous endeavours have made to the materials 

 for finally deciding upon the position which the extinct Mosasaurians 

 held in the cold-blooded, air-breathing division of Vertebrates. 



Discussion. 



Prof. Seelet could not exactly recognize the Lacertilian type in 

 the Mosasaurian limb, and thought that the proximal segment in 

 the fossil, which was more likely to indicate affinities than the distal 

 modifications, was not Lacertilian. He urged that while Prof. 

 Owen had admirably elucidated the Lacertian affinities of these 

 animals, they did not therefore of necessity belong to the Order 

 Lacertilia ; and, in view of structural differences in all parts of the 

 skeleton, he considered it was at least a convenience to place them 

 in a distinct Ordinal Group, as suggested by Prof. Cope. 



Prof. Duncan thought that Prof. Cope had laid too much stress upon 

 and ascribed ideal value to characters which could not be regarded 

 as being, at best, of more than secondary value. He insisted 

 on the important character of the possession of limbs by the Mosa- 

 saurs as strikingly distinguishing them from the Ophidia. He re- 

 marked further on the absence of a sternum, and on the Lacertian 

 affinities of the sacrum, although he admitted the teeth had Ophidian 



Op. cit. p. 300. 



