and certain pleurodiral turtles have saddle-shaped articulations. In no other order of reptiles are the variations so great as in this.
Fig. 75. Dorsal vertebra of Diplodocus (Saurischia). After Hatcher. One tenth natural size.
The pleurocoelous (Fig. 75) presacral vertebrae of the sauropod dinosaurs are peculiar in having large cavities in the centra, separated by a median partition, with an oval or round orifice at each side. Not only is the centrum thus lightened in these dinosaurs, but the arch is curiously strengthened by plates and buttresses. Certain other South African reptiles (Tamboeria) also have pleurocoelous vertebrae. It is supposed that this hollowness and lightness of the cervical and dorsal vertebrae, correlated with the otherwise solid or cancellous skeleton, served to keep the body erect in water, their natural habitat in wading or swimming.
Except in the snakes and legless lizards, where but two regions are recognized, the caudal and precaudal, the spinal column of reptiles is divisible into cervical, dorsal, sacral, and caudal regions, and sometimes lumbar also, as in mammals. The cervical region is that in front of the shoulder-girdle, the dorsal that between the shoulder and hip girdles, the sacral that which supports the hip girdle, and the caudal that of the tail.
We may hardly venture to guess as to the primitive number of vertebrae in reptiles. We are quite sure that there has been an increase in number in some, a decrease in others. The land temnospondylous amphibians that we know have but one real cervical vertebra, the so-called atlas, twenty-two to twenty-five dorsals, one or two sacrals, and a short or moderately long tail. Trimerorhachis, an aquatic Lower Permian temnospondyl, has thirty-one precaudal vertebrae and no differentiated sacrals. The earliest reptile that we know, Eosauravus, subaquatic in habit, had at least twenty-four or twenty-five pre-
