328 ON SIMILTTTJDES OF THE BONES IN THE ENALIOSAUEIA. 



allel with them at the hinder part ; the palate is mesially open. 

 There is no resemblance in the vertebral column, 



§ 8. The Urodelan Characters of Plesiosaurus. 



The skull of the ilenopome or of the Salamander has no striking 

 resemblance to Plesiosaurus in any region ; while the absence of 

 a prenasal extension, the confluence of the orbit and temporal 

 fossa, the two occipital condyles, the median bone (parasphenoid) 

 at the base of the skull, the absence of covering bones from the 

 quadrate bone, and the absence of socketed teeth would readily 

 distinguish them. 



The vertebrae agree in having the centrum biconcave ; but the 

 Urodelan centrum is much longer, and its neural arch wants the 

 long compressed neural spine of Plesiosaurus. The cervical ver- 

 tebrae are veiy few, and have transverse processes, which are not 

 seen in Plesiosaurus, while the amphibians have usually no atlas. 



The dorsal vertebrae agree with Plesiosaurs' in having transverse 

 processes, Avhich, however, are not given off exclusively from the 

 neural arch, are compressed and directed obliquely backward. At 

 their termination in some types the parts contributed by the 

 neural arch and by the centrum divide so as to make a double at- 

 tachment for the rib. The costal ribs differ in being double-headed 

 and extremely short, wdiile no sternal or abdominal ribs are deve- 

 loped. The caudal vertebrae differ additionally in having their 

 chevron bones anchylosed to the base of the centrum. 



The pectoral girdle differs in having the scapula and coracoid 

 united in one ossification, and in the coracoid portions being di- 

 vided by a wide cartilage. The scapulae have no tendency to 

 approximate anteriorly. 



The ilium is not unlike that of Plesiosaurus. The bone inferior 

 to it is usually named the ischium ; but if these bones were re- 

 volved forward so that the median line became anterior, and the 

 bones met mesially at their posterior borders, then they would 

 have the position, as they already have the shape, of the pubic 

 bones of Plesiosaurus. 



In the limbs there is some resemblance ; but the shaft of the hu- 

 merus and femur is not sufficiently expanded, and the proximal tro- 

 chanter should not have been severed from the head of the bone, the 

 forearm and foreleg are too long ; and the ulna and tibia are not re- 

 niform ; the carpals and tarsals are more irregular in size, and have 



