Williston.] Mosasaurs: 135 



changing the shape of the centra. The vertebra? of Clidastes 

 may always be readily distinguished from those of the other 

 genera by their greater slenderness, and especially by the pres- 

 ence of a more or less complete zygosphene. The vertebrae of 

 Platecarpus and Tylosaurus do not present any very tangible dif- 

 ferences from each other, and it is often difficult, if not impos- 

 sible, to distinguish them generically, unless considerable series 

 are examined. 



Only three divisions can be distinguished : cervical, dorsal, and 

 caudal, and even the cervical do not show clear distinctions from 

 the dorsal. For convenience, however, the true thoracic verte- 

 bra? — that is, those bearing long ribs — ma}?" be distinguished 

 from the posterior dorsal or lumbo-dorsal vertebra?, or those bear- 

 ing short ribs. The vertebra? themselves, however, will not en- 

 able one to place them correctly in their series, except by direct 

 comparison with others of the series. In the posterior lumbo- 

 dorsals the zygapophyses are weaker and less developed, and the 

 transverse processes smaller. At the beginning of the caudal 

 series there is an abrupt change, the short costiferous processes 

 giving place to flattened, elongate processes that are non- 

 costiferous. The non-chevron-bearing vertebra? with these 

 simple processes vary in number, in our genera from five to 

 seven, and have been called pygial by the writer and E. C. Case. 

 The distinguished paleontologist, Doctor Dollo, has expressed a 

 doubt of the nature of these vertebra?, contending that some or 

 all of them are true lumbar vertebra?, as they had all been pre- 

 viously considered. I feel yet more assured that they are true 

 caudal vertebra? — that is, situated back of the pelvis — and have 

 so described and figured them in the restoration of the animals. 

 In Varanus, for instance, where there is a true sacrum, the stout 

 ilia are directed upward and backward to unite by synchondrosis, 

 extending nearly as far back as the posterior part of the ischium. 

 This places the ischial symphysis below the sacrum and in front 

 of the chevron-bearing caudals, and leaves the opening for the 

 pelvis entirely unrestricted. The number of the vertebra? back of 

 the sacrum without chevrons in Varanus is but two, as more are 

 not needed, in reality corresponding to four in the Mosasaurs, 



