COPE ON OWEN ON PYTHONOMORPHA. 307 



tween this part of the structure of these animals and some of the 

 Erycid and Pythonid serpents than Professor Owen admits in his paper. 



Characters of vertebras furnish the points of the essay from the nintJh 

 to the thirteenth. As I only cite a single vertebral character in my list 

 of those of the order, most of Professor Owen's arguments on this head 

 are irrelevant to my conclusion. I will, however, briefly review them. 

 But firstly as to the one to which I have attached weight, — the absence 

 of a sacrum. Professor Owen is unfortunate in his reasoning against 

 the use of this feature as an ordinal definition. He says : — " The absence 

 of a sacrum does not affect the mammalian grade of the Sirenia or Ce- 

 tacea, so neither does it the lacertian nature of the Mosasaurians"'! 

 Here is committed the extraordinary oversight of comparing the rank 

 of orders in a class with the rank of the subdivisions of an order amon^ 

 themselves. Professor Owen should have concluded the sentence with 

 '^ so neither does it the reptilian nature of the Mosasaurians ", in wMch 

 case he would have been correct. The cases of the mammalian orders 

 and that of the Fythonomorpha as orders of classes are indeed parallel. 

 The absence of a sacrum is an important definition of the orders in th© 

 one case as in the other. 



Of other vertebral characters I only mention two. Professor Owen 

 cites the numerous hypapophyses of certain snakes as evidence against 

 Ophidian affinities of Mosasauroids, but, as usual, selects those whick 

 have the largest numbers for comparison instead of those where the 

 number is reduced. In the majority of non-venomous and Colubroid 

 serpents, the hypapophyses are confined to the anterior part of the col- 

 umn, leaving the other vertebrae either smooth or not protuberant be- 

 yond the horizontal inferior line; e. g., Xenodon, Seterodon. The only 

 exception to this rule is seen in the fresh-water snakes (Homalopsidw), 

 where the hypapophyses are numerous. The character is not, however^ 

 ordinal in any case. 



In discussing the other vertebral character, the structure of the atlas 

 and axis, I am charged with the failure to recognize the homology of 

 the odontoid process with the centrum of the atlas. There is no ground 

 for this charge; and as Professor Owen finds no characters which dis- 

 tinguish these parts* from the corresponding ones in Colubroid snakes, 

 I leave it. 



As the fourteenth point, the significance of the structure of the teeth 

 may be considered. I have already adverted to the wide difference in 

 the mode of support of the crowns by the jaws from that which is uni- 

 versal in the Laeertilia. Professor Owen repeats a former dictum, that 

 this kind of attachment " is a feature of resemblance to the lacertians 

 called acrodont". Now even the term "resemblance" can hardly be ad- 

 mitted; and as to homology between the two kinds of dental attachment, 

 there is none. Says Professor Gervais, in the Zoologie et PaleontoJogie 

 Frangaises, tome i., page 262, in describing some teeth which he refers to 

 Liodon, in a note,—" C'est h tort que I'on decrit les dents des Mosasaures 



