78 



tebra? above the pelvic arch were furnished with elongate, subcylindric dia- 

 pophyses. 



Dr. Turner, the discoverer of the original specimen, having made a sec- 

 ond careful search and renewed excavations at the original locality, failed to 

 find any bones which can be assigned to humerus, ulna, radius, carpus, or 

 phalanges, or similar elements of the hind limbs. The pelvic and scapular 

 arches were further completed, and an additional number of ribs obtained. 

 The glenoid cavities are rather angular, and both were filled with solid 

 argillaceous matrix. The acetabula are not cuplikc, but merely exposures 

 of the narrow plane extremities of the pubes and ischia ; they were covered 

 with thin layers of gypsum ; the pieces of the ilia were found imbedded in 

 the mass of matrix which occupied the pelvic arch. 



This genus is well distinguished from Plesiosaurus by the peculiarity of 

 the scapular arch. The mesosternum appears to be wanting, and the clav- 

 icles and coracoids form a breastplate. If the claviculus was ever united 

 with the scapula, as in Plesiosaurus, no evidence of it can be seen in the speci- 

 men ; it is also broader and more extended anteriorly. 



Differences from other Sauropterygia. — The only genus which it is nec- 

 essary to compare with the present one is Cimoliasaurus. The following 

 may be noted as generic distinctions : the series of cervicals rapidly diminishes 

 in Cimoliasaurus in absolute size and in relative length of the vertebrae, which 

 are not compressed. In the present genus, they maintain a similar and in- 

 creased length for a considerable distance, diminish in length very gradually, 

 and are much compressed. The diapophyses erf the dorsal vertebrae, as they 

 descend in Cimoliasaurus, continue well developed until they attain the infe- 

 rior planes of tlie centra, and have there a downward direction. In Elasmo- 

 saurus they cease while yet on the middle of the centrum, and are replaced 

 by pits throughout the remainder of the length. 



The neufal canal is everywhere markedly larger in Cimoliasaurus. 



The American genera of Elasmosauridce may be compared as follows: 



Posterior cervical vertebrae without diapophyses : 



Cervicals long, compressed; neck very elongate Elasmosaurus. 



Posterior cervical vertebrae with diapophyses : 



Cervicals cpiadrate, short, depressed, transverse, rapidly 



diminishing in size, hence the neck short Cimoliasaurus. 



