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Show 1891.] HELODERMA HORRIDUM AND H. SUSPECTUM. 113 of these five parts had co-ossified." I can, however, now confidently affirm that Dr. Shufeldt has been deceived in his examination. The atlas-ring of Heloderma is formed, as in all Reptiles, of three pieces, a ventral and two dorso-lateral. The presence of five elements in the atlas-ring, if such had been the case, would have entirely upset the current view on the morphology of the vertebral column, which holds the said ring to be formed of the neural arch of the atlas and the proatlanto-atlantic hypapophysis or intercentrum ; the centrum of the atlas being either free behind the ring or fused with the centrum of the vertebra following (odontoid process of the epistropheus). I regard the views held by Cope \ Baur2, and Credner3 on the morphology of the vertebral column, based as they are on the evidence of the primitive structure afforded by many Stegocephalians, as thoroughly sound, and borne out by everything we know of the structure of recent and fossil Reptiles. The vertebrae of Reptiles are composed of the following elements:-Neural arch (neurapophyses), centrum, and intercentrum (hypapophyses, subvertebral wedge-bones, chevrons). No Reptile shows an exogenous hypapophysis together with an autogenous hypapophysis, wedge-bone or chevron on the same centrum ', and the continuity of the series of intercentral autogenous hypapophyses throughout the vertebral column, together with the gradual passage of the wedge-bones into the chevrons, is clearly exhibited in Sphenodon and the Geckos. The homology of the cervical hypapophyses with the chevrons is further manifested by such Squamata as have the chevrons attached to a single centrum, viz., the Anguidce, Varanidce, and Mosasauridce, having the cervical hypapophyses likewise on the centrum ; whilst those having the chevrons intercentral, viz., the Agamidce, Lguanidce, Lacertidce, most Scincida;, Chamaileon-tidce, & c , have also the cervical hypapophyses so disposed. In Sphenodon and Geckos, in which the branches of the anterior chevrons are united at the base, the hypapophysis anterior to the first chevron is single, but when the chevrons are V-shaped the hypapophysis preceding them is paired. Such is the case in Heloderma, and I have observed the same thing in many other Lizards, where these little bones bear much resemblance to the cervical hypapophyses of many Chelonians, or of Lacerta agilis, as figured by Leydig (Deutschl. Saur. pi. iv. fig. 53). I believe, however, that paired autogenous hypapophyses have not been recorded before in the caudal region of Lizards. The paired inferior processes of the caudal vertebrae of Snakes must be likewise regarded as homologous i Amer. Nat. 1878, p. 327, and Tr. Am. Philos. Soc. (2) xvi. 1888, p. 243. 2 Biol. Centralbl. vi. 1886, p. 332. 3 Zeitschr. deutsch. geol. Gres. xiii. 1890, p. 260. 4 Hulke (P. Z. S. 1888, p. 422) states that in the cervical vertebras of Tra-chydosaurus rugosus " the ' intercalary' or intercentrum coexists with a genuine [exogenous] hypapophysis;" but I have been unable to find any substantiation of this statement on a specimen of that Lizard in the College of Surgeons, which I have had especially cleaned for examining this point. It is possible that the part termed by Hulke intercentrum is an epiphysis of the hypapophysis such as is so well developed in Varanus and Mosasauria. PROC. ZOOL. Soc-1891, No. VIII. 8 |