130 WATER REPTILES OF THE PAST AND PRESENT 



the crocodiles, the proganosaurs had by no means lost their land 

 proclivities. 



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 noto- 

 chord. The pelvis also is of the old-fashioned type, that is, with- 

 out 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, feed- 

 ing 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 liz- 

 ards, 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, remind- 

 ing one of those two other groups of swimming reptiles, the dolicho- 

 saur 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 



