THE PLESIOSAURIA. 181 



costji,! articular surface is cut by the neurocentral suture, as 

 the last of the cervical series. 



The two anterior cervical vertebrce, as thus defined, con- 

 stitute the atlas and axis, and are frequently anchylosed to- 

 gether. The center of the other cervical vertebrae have slightly 

 concave anterior and posterior surfaces ; well-developed neural 

 arches ; anterior and posterior oblique processes, or zyga» 

 pophyses, of the ordinary character ; and stout, but somewhat 

 short, spinous processes. The centrum presents, upon each 

 side, an oval rugose pit, sometimes more or less divided 

 i.ito two facets. This is the costal articular surface, which 

 has been already adverted to. Into it fits the thickened head 

 of a costal rib, which may have corresponding facets, but is 

 otherwise undivided. The rib is continued backward into a 

 short and straight body, and the angle, or the part at which 

 the neck and the body of the rib join, is produced forward, so 

 that the cervical ribs of the Plesiosauria have a strong general 

 resemblance to those of the GrocodUia. In the posterior part 

 of the neck and the anterior part of tlie dorsal region, the ribs 

 become somewhat longer, and lose their anterior processes, 

 gradually acquiring the rounded and curved form of ordinary 

 ribs. Their proximal ends remain simple, and the facets, with 

 which they articulate, become raised, and thrown outward, as 

 transverse processes, developed from the arches of the verte- 

 brae. (Fig. 68, C.) 



In the anterior dorsal vertebrae, these transverse processes 

 rapidly acquire their full length; and they are continued un- 

 der this form, descending somewhat lower upon the arches of 

 the vertebrae toward the sacrum, to the end of the dorsal re- 

 gion. The neural spines acquire greater length, the zyga- 

 pophyses are well developed, and the articular surfaces of the 

 centra retain the form which they possessed in the cervical 

 region. There are usually between twenty and twenty-five 

 dorsal vertebrae. The sacral v^ertebrae are two, and resemble 

 the others, except that the sacral ribs are large and broad for 

 the attachment of the ilium. The caudal vertebrae, usually 

 between thirty and forty in number, become, as usual, reduced 

 to little more than centra at the end of the tail ; but, in the 

 fore part of the tail, they have well-developed spines and ar- 

 ticular processes, with ribs which become anchylosed to the 

 bodies of the vertebrae, only late in life. Well-developed 

 chevron-bones are attached between the ventral margins of 

 successive centra of the caudal vertebrfie. 



As has been mentioned, there appear to be no sternal ribs, 



