Page:The Osteology of the Reptiles.pdf/283

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Subvolant, arboreal, terrestrial, burrowing, subaquatic, or marine reptiles from a few inches to about forty feet in length; quadrupedal, bipedal, or limbless; herbivorous, insectivorous, or carnivorous. Brain-case in front of proötics more or less membranous. Lacrimals small or vestigial. Posterior arcade sometimes absent. Mandibles usually united by suture. Vertebrae procoelous, except in the Geckonidae and Uroplatidae; not more than two sacral vertebrae. Clavicles and interclavicle rarely absent. No entepicondylar, but usually an ectepicondylar foramen in humerus.

This group is often given an ordinal rank, equivalent to the Ophidia or even to the Pythonomorpha, but the ultimate distinctions between them are almost trivial, as will be seen, and in many legless burrowing lizards the skull structure mimics that of the snakes. More than eighteen hundred species are known, distributed widely throughout the world, usually classed in about twenty families and numerous genera.

Because of their predominantly terrestrial habits, but few remains of lizards are found in the rocks, aside from the more aquatic or marine types. Only about fifty genera of extinct forms have been described and less than one hundred species, and the greater majority of those are for the most part fragmentary and incomplete, so much so that their systematic positions are very often uncertain and provisional. Doubtless they have had a long and abundant geological history from very remote times, but of the true land lizards almost nothing is known throughout the Mesozoic. But few positive characters are distinctive of the group, though many negative ones are. The mandibles are usually suturally united in the middle, but a few forms have them ligamentously attached. The presence of legs is not distinctive, though at least a vestige of the pectoral girdle remains. The more or less open brain-case in front is perhaps the most diagnostic, only partially enclosed by the more or less vestigial postoptics ("alisphenoids," "postorbitals"). However, in the Amphisbaenia even this character is doubtful, and in the mosasaurs a distinct descending plate from the parietals resembles that of the snakes,