Their vertebrae, as would be expected in such old reptiles, are quite primitive in structure, that is, they are deeply concave in each end, probably being perforated for the remains of the notochord. The pelvis also is of the old-fashioned type, that is, without an opening or vacuity between the bones below. The shoulder bones are old fashioned too. The shoulder-blade, especially, shows a decided adaptation to water life in its short, fan-like shape, very much like those of the mosasaurs, ichthyosaurs, whales, etc. Just why swimming animals should have short and broad shoulder-blades has not yet been explained, but doubtless they afforded better attachment for those muscles used more especially in swimming. The ribs are remarkably flat and heavy, and were not very firmly attached to the vertebrae. Heavy ribs are unusual among free swimming animals, but do occur in the modern sirenians, which live on the bottoms of shallow bays, etc., feeding upon plants. We may perhaps infer from this peculiar structure of the ribs that the proganosaurs lived more on the bottoms of shallow waters, feeding upon such fishes or invertebrates as they could capture, coming to the surface to breathe from time to time. Possibly they sought the shores for safety from their enemies, as do the Galapagos lizards, figured on p. 142; and doubtless they laid and hatched their eggs on land. A character which suggests that the proganosaurs lived only in the shallow waters is the elongated neck, reminding one of those two other groups of swimming reptiles, the dolichosaur lizards and the nothosaurs of the Sauropterygia, the only known reptiles besides the plesiosaurs having an abnormal number of neck bones. Still more suggestive of shallow, fresh-water habits is the absence of eye bones, as in the modern crocodiles. The long snout, with the long and slender teeth, and the position of the external nostrils far back near the eyes, together with the flattened and long tail and the webbed feet, are sufficient proof of expert swimming habits. The legs still functioned more or less for the support and propulsion of the body on the land, and they probably were only of slight service in the water. The alligator swims sinuously with its front legs collapsed and extended by the side of the body; its hind legs are used more as propellers, with the knee flexed and the feet turned outward and expanded. The legs of the proganosaurs doubtless were used in the same way, as shown in the restoration, which has been modified from the original of Dr. McGregor in accordance with this probable use of the legs.