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Show 1891.] HELODERMA HORRIDUM AND H. SUSPECTUM. 117 Although fully admitting the name Pythonomorphal to have been ill chosen, I cannot but agree with Prof. Cope in maintaining the Mosasaurs as a suborder, if only for the hyperphalangy of their limbs 2 and the type of their dentition, the large osseous bases which bear the teeth being inserted in a groove of the jaws, a feature which may be regarded as midway between the thecodont and acro-dont types; whereas the Monitors and Heloderms belong to the pleurodont type. The Helodermatidce on the other hand are true Lacertilia, more closely related, in m y opinion, to the Anguidce than to the Varanidce. And although there are undoubtedly many points common to the Monitors and the Mosasaurs, I hold that Dr. Baur is mistaken in proposing to revert to the Cuvierian views o the affinities of the large extinct marine Reptiles. Dr. Baur says : " It is evident that the Mosasauridce are very closely related to the Varanidce. They simply represent highly specialized aquatic forms." Does this mean that limbs so strongly specialized as those of the Monitors can have been modified into the paddles of the Mosasaurs ? A glance at the figures (see fig. 6, p. 118) suffices to refute such a theory. But we can perfectly well conceive the hind limb of a Doli-chosaurian becoming modified into the said paddle ; and I can see no reason for not regarding these Cretaceous Lizards as the progenitors of the Mosasaurs, and at the same time of the true Lacertilia of which the Pleistocene and recent Varanidce are a family. This view is besides in accordance with the suggestion made by Dollo3, that the progenitors of the Mosasaurs must have possessed the zygosphenal articulation. The Order Squamata may very well be divided into the following five Suborders, merely with regard to the structure of the limbs and vertebral column:- A. Pectoral arch or its rudiments present. Caudal hypapophyses forming chevrons. * I. Dolichosauria. 15-17 cervical vertebrae. Extremities (Fig. 6 A, p. 118) archaic, i. e., approaching the Batrachian type. II. Pythonomorpha. 9 or 10 cervical vertebrae. Extremities (Fig. 6 B, p. 118) paddle-shaped, with hyperphalangy. III. Lacertilia. 8 or 9 cervical vertebrae. Fibula reduced proximally ; fifth metatarsal reduced in length and strongly modified (Fig. 6 C, p. 118). IV. Rhiptoglossa. 5 cervical vertebrae. Extremities pincer-shaped ; all the metatarsals reduced in length and strongly modified (Fig. 6 D, p. 118). B. N o trace of pectoral arch. Caudal hypapophyses disconnected distally. V. Ophidia. 1 Pythonomorpha, Cope, 1869, = Mosasauria, Marsh, 1880. 2 At least three phalanges in digit I. 3 Bull. Soc. Beige Geol. iv. 1890, p. 167. |
