BRfTISH FOSSIL REPTILES. 51 



tebral 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 cen- 

 trum below, — the spines here remaining apparently throughout 

 life unanchylosed to the neurapophyses. This condition of the 

 upper vertebral 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 neurapophyses at an earlier period. 



The hsemapophyses 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 inferior surface of great part of the abdomen, forming there 

 the sternal or abdominal 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 hsemapophyses 

 are in like manner elongated transversely, and their spine is in- 

 troduced and modified to form a third mesial rib-like bar, con- 

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

 the h8emapophyses_, and completing the osseous cincture of the 

 abdominal viscera. 



The tail in the Plesiosauri is relatively much shorter than 

 in the Ichthyosauri, and there is an obvious reason for the 

 curtailment of this part of the animal; for in the Plesiosauri, 

 the length and mobility of the neck renders a special develop- 

 ment of the tail for producing the lateral movements of the 

 head unnecessary. 



The bodies of the vertebrae, in most species of Plesiosaurus, 

 are traversed vertically by two vascular canals, which lead from 

 the medullary or spinal canal to the inferior surface of the cen- 

 trum, where they terminate each by an orifice, and sometimes 

 by two orifices, on each side the middle line. These orifices 

 are not, however, a constant character of the genus Plesiosaurus, 

 neither are they peculiar to this genus, being present in the ver- 

 tebrae of the Cetacea, as well as in those of other Sauria. 



In a section of the vertebral centrum of a Plesiosaurus, the 

 osseous texture for some lines near the anterior and poste- 

 rior articular surfaces is denser than in the rest of the vertebrae, 

 and the direction of the laminae and fibres is vertical : in the 

 intermediate portion the laminae are horizontal. 



E 2 



