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Show 116 MR. G. A. BOULENGER ON THE OSTEOLOGY OF [Jan. 20, comparison of the vertebral columns of the two species of Heloderma, we have merely to notice the following differences : - A short rib is present on the third cervical in LI. horridum, which is absent in //. 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 Helodermatidce between the Anguidce and the Varanidce, 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 ossifications1, 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 was covered with large osseous shields, symmetrically arranged and highly ornamented, resembling in this respect the modern Heloderma." In G. rugosus " the prefrontal and post-frontal bones approach each other above the orbit." The agreement with the Varanidce is in the arrangement of the bones of the palate and the presence of descending laminae of the frontals forming a bridge under the olfactory lobes of the brain (a character shown by the Geckonidce, Uroplatidce, Eublepharidce, and Snakes2). Apart from the secondary character of the presence of grooves, the teeth of Heloderma resemble those of Anguis and Varanus. 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 importance, 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'ss proposal to group the Varanidce, Mosasauridce, and Helodermatidce together as follows :- T7I f Varanidce. r, f VARANOIDEA < j,. ., PLATYNOTA. J [ Mosasauridce. I H E L O D E R M A T O I D E A . . .. Helodermatidce. 1 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. 8 Science, xvi. 1890, p. 262. |