SQUAMATA 



159 



the only ones among water reptiles that would be dangerous and 

 offensive to man, were they all living today. 



For a long time it was thought that the mosasaurs had no 

 breast bone, and that, in consequence, the front part of the thorax 

 Was expansible. Under this assumption the mosasaurs would have 

 been much more snake-like in habit than they really were. The 

 loose construction of the jaws doubtless permitted the swallowing 

 of prey of considerable size, and the inference was that they habitu- 

 ally preyed upon animals of large size. A snake will often swallow 

 a frog of larger- diameter than its own body, the flexible jaws and 

 loosely connected ribs permitting it to pass to the abdominal 

 cavity. But the unyielding ring formed by the anterior ribs .con- 

 nected with the breast bone in the mosasaurs, as in other lizards, 

 conclusively proves that large animals could not have been swal- 



FiG. 77. — Platecarpus; pelvis, from below: p, pubis; il, ilium; is, ischium 



lowed whole by the mosasaurs. In several instances the fossilized 

 stomach contents, composed chiefly or wholly of fishes, have been 

 found between the ribs of mosasaurs, and in none were the fishes 

 more than two or three feet in length, though the reptiles were 

 from sixteen to twenty feet long. Possibly the largest mosasaurs 

 those thirty or thirty-five feet in length, might have captured and 

 swallowed fishes six or seven feet long, but in all probabihty their 

 usual prey was of smaller relative size. 



The very loose construction of the pelvic bones, those to which 

 the hind legs are articulated, is an evidence of more complete 

 adaptation to water Ufe than was or is the case with any other 

 water air-breathers except the ichthyosaurs and cetaceans. The 

 sacrum had entirely lost its function as a support to the pelvis 

 and had disappeared, that is, the vertebrae composing it had become 



