523 Richard Owen, Esq., ow the 



boldly out as a true transverse process from the upper side of the base of each 

 neurapophysis. At the sacral vertebrsB, however, the transverse processes 

 subside to the level of the neurapophyses ; and, as the caudal vertebrae re- 

 cede from the trunk, the articular surface, which, as in the neck, represents, 

 or is in the situation of the transverse process, gradually descends and passes 

 from the neurap(>physis to the side of the centrum ; but it is not divided by 

 the longitudinal groove which characterizes the costal surface in the neck. 



This groove is more marked in some than in other species of Plesiosaurus ; 

 and I have seen Plesiosaurian vertebrae undoubtedly cervical, in which no 

 trace of it was visible. 



The neurapophyses are commonly unanchylosed with the vertebral centres 

 in any part of the spine, and in some instances throughout the cervical, and 

 at the anterior part of the dorsal region, the neurapophyses have appeared 

 to be joined each by an articular surface to the spine above, as they are 

 to the centrum below, — the spines here remaining, apparently throughout 

 life, unanchylosed to the neurapophyses. This condition of the upper ver- 

 tebral elements is rarely seen in any cold-blooded vertebrate, and never in 

 the warm-blooded classes. 



In those parts of the spine, where the vertebrae enjoyed less mobility upon 

 each other than in the neck, the spines become anchylosed to the neurapo- 

 physes at an earlier period. 



The haemapophyses co-exist with the ribs or paravertebral elements, in the 

 caudal region of the spine, but they continue throughout life to be unattached 

 by bone, either to the centrum above or to each other below ; and here also 

 their spine is not developed, and consequently no true chevron-bone is formed 

 in the Plesiosauri. The heemapophyses are also continued along the infe- 

 rior surface of a great part of the abdomen, forming there the sternal or ab- 

 dominal ribs ; and just as the neurapophyses are developed in the transverse 

 direction to protect the expanded cerebral masses in the cranial region, so 

 here the haemapophyses are in like manner elongated transversely, and their 

 spine is introduced and modified to form a third mesial rib-like bar, con- 

 necting, however, as usual, the lower or distal extremities of the haemapo- 

 physes, and completing the osseous cincture of the abdominal viscera. 



The tail in the Plesiosauri is relatively much shorter than in the Ichthyo- 

 sauri, and I have not observed in any example, that partial dislocation which 

 seems to indicate that it supported a fin in the Ichthyosauri. There is an 

 obvious reason for the shortness and simplicity of this part of the animal; in 

 theP/eszosaMn,thelength and mobility of the neck renders a special apparatus 

 in the tail for producing the lateral movements of the head unnecessary. 



