708 PROF. OWEN ON TI1E RANK AND AFFINITIES IN 



of Python ; and the pair of tubercles rapidly elongate in the caudal 

 series to the proportions shown in figure 42 of plate xix. of the 

 above-cited Monograph. Moreover, in certain anterior caudals of 

 Python regius, the di-pleurapophysis is bifurcate (fig. 41, pi. xix., 

 Monogr. cit.). 62 out of the 70 caudal vertebrae of Python tigris 

 have each the pair of parallel exogenous hypapophyses. 



In Coluber natriv hypapophyses exist upon all the vertebrae, of 

 which there are 170 of the neck and trunk, and 40 caudal ; in no 

 Ophidian have I found a chevron- bone. 



Thus there are well-marked characters distinguishing the Ophidian 

 from the Lacertian vertebrae, which should be brought to bear upon 

 the question of the affinities of the Mosasaurians. 



The latter, as is well known, agree with both orders and with 

 proccelian crocodiles in having the fore surface of the centrum con- 

 cave, the hind one convex. The two vertebrae which follow the 

 axis have a single hypapophysis (fig. 23, hy) the terminal portion of 

 which (lis) is distinct, seemingly in the state of an epiphysis. 

 Cuvier's figure 1, pi. xix., shows the rough concave surface of this 

 single hypapophysis, to which the free portion * was attached ; thus a 

 smaller portion of the hypapophsis is free than in Aniblyrhynchus, 

 no portion being free in Python or other serpents. Beyond the fifth 

 of the anterior vertebrae, including axis and atlas, in Mosasaurus, 

 the hypapophysis, after becoming reduced in size, disappears, and is 

 not again developed f . 



The diapophysis, in certain trunk-vertebrae, shows a terminal sur- 

 face for the rib. Prof. Cope, in his restoration of a Mosasaurus 

 (pi. lv.), gives it 37 pairs of ribs, all terminating freely, as in 

 Ophidia. But I do not find the authority for this in the special 

 descriptions of the fossils described by him in situ J. 



The diapophyses, in Mosasaurus Hoffmanni, decrease in vertical 

 extent as the vertebrae recede from the neck, and lose, at an unde- 

 termined part of the trunk-series, their zygapophyses ; the centrums 

 also lose in length. The diapophyses exchange their vertical for 

 transverse breadth, and gain in length, apparently in the part of the 

 trunk where the free pleurapophyses cease ; and here, in M. Hoff- 

 manni, the centrum assumes a triangular form. In the tail, where 

 a haemal arch (chevron bone) is articulated, as in Amblyrhynchus, to 

 a pair of hypapophyses near the hind surface, and coexists with dia- 

 pophyses, the centrum becomes pentagonal ; where the diapophyses 

 disappear and the spine of the haemal arch is prolonged, the centrum 

 becomes vertically ovate ; further back the haemal arch coalesces 

 with the centrum ; its spine is grooved in front, and exceeds in 

 length the neural arch and spine (Cuv. pi. xix. fig. 6), but it gra- 

 dually shortens and finally disappears ; the centrum with a small 



* A homologue of haemal spine articulating to a hypapophysis. 



t Prof. Cope states that "the dorsals have no hypapophysis " (p. 123), in 

 which distinction from Ophidians his supplementary species of Mosasaurians 

 agree with the type species. 



| He admits : — " I do not possess any specimen with complete vertebral 

 column " (p. 124). 



