116 MR. G. A. BOULENGER ON THE OSTEOLOGY OF [Jan. 20, 



comparison of the vertebral columns of the two species of Seloderma, 

 we have merely to notice the following differences : — ^A short rib 

 is present on the third cervical in H. horridum, vyhich is absent in 

 H. suspectum ; the neural spines are more elevated in the middle and 

 posterior portion of the dorsal region in H. horridum, specimens of 

 the same sex, of course, being compared. The neural spines are 

 much more developed in the male. 



Systematic Position of Heloderma. 



That Heloderma is the type of a distinct family of Lizards is now 

 universally admitted, but views differ as to its position in the system. 

 The place of the Selodermatidce between the Anguidce and the 

 VaranidcB, which I assigned to them in 1884, is, I still think, the 

 most natural. They agree with the Anguidce. in the structure of the 

 tongue and the presence of dermal ossifications \ and it is probable 

 that direct comparison of them with the numerous remains from the 

 Eocene of Wyoming, described by Marsh (Am. Journ. (3) i. 1871, 

 p. 456, and iv. 1872, pp. 302 and 305) as Glyptosaurus, would 

 reveal their closer resemblance to these than to any of the existing 

 members of that family. Marsh remarks of his Glyptosaurus that 

 " the head vras covered with large osseous shields, symmetrically 

 arranged and highly ornamented, resembling in this respect the 

 raoAexM Heloderma." In G. rugosus " the prefrontal and post- 

 frontal bones approach each other above the orbit." 



The agreement w^ith the Varanidae is in the arrangement of the 

 bones of the palate and the presence of descending laminse of the 

 frontals forming a bridge under the olfactory lobes of the brain (a 

 character shown by the Gechonidce, Tlroplatidce, Eublepharidce, and 

 Snakes ''). Apart from the secondary character of the presence of 

 grooves, the teeth of Heloderma resemble those of Anguis and Va- 

 ranus. The most important character which differentiates Heloderma 

 from all other Lizards is the presence of a bony postorbital arch, 

 combined with the absence of a zygomatic arch. The absence of a 

 transverse limb to the interclavicle is not of more than generic impor- 

 tance, as it occurs also in an Agamoid, Lophura, and the reverse 

 modification, viz., the suppression of a longitudinal limb, in an 

 Iguanoid, Phrynosoma. 



The latest attempt at fixing the systematic position of Heloderma 

 is Baur's ^ proposal to group the VciranidcB, Mosasauridce, and Helo- 

 dermatidce together as follows :— 



p r Varanoidea. . j ^^^ranidcB 



rLATYNOTA. J \ Mosasauridts. 



I Helodermatoidea. . . . Helodermatidce. 



^ Through a printer's error, it is stated in my ' Catalogue of Lizards,' vol. ii. 

 p. 266, that the dermal plates " are provided with a system of fine tubercles." 

 Tubercles is so obviously a misprint for tubules that I should not have thought 

 it worth while to correct the statement here but for the fact that it has been 

 repeated in several recent palaeontological works. 



^ But not by the Mosasaurs. 



^ Science, xvi. 1890, p. 262. 



