62 Field Columbian Museum — Geology. Vol. II. 



Tin- increase in length of the cervical vertebrae is slight, as will be 

 stLii from the table of measurements. The articular margin of the 

 vertebrae, throughout both the cervical and dorsal series, is sharp, 

 not rounded for a continuation of the cartilaginous surface, as is so 

 often the case among plesiosaurs. The ends, as seen in the third 

 vertebra, are distinctly though not deeply concave. The ventral 

 vascular foramina, so characteristic of plesiosaur vertebrae, appear to 

 be wanting throughout the whole series, nor is there the slightest 

 indication of a median ridge and lateral grooves. The lower margin 

 of the rib articulation approaches the ventral plane in the early 

 ■cervicals, but ascends somewhat on the sides of the vertebra in the 

 last cervical. 



Twenty-two dorsal vertebrae are preserved in natural sequence. 

 Because of the decrease in the length of the transverse process in the 

 late dorsals, it does not seem probable that more than eight or ten 

 presacral vertebrae could be missing, making altogether about thirty, 

 the usual number of dorsal vertebrae. The length of the centra 

 increases more rapidly, though gradually, in the early dorsals, and 

 then remains constant throughout the remainder of the series. The 

 centra are deeply concave below and on the sides, expanding out- 

 wardly on the sides above to meet the sutural surface for the arch. 

 The surface of the centra is nearly smooth, with but slight indications 

 of crenulations before the articular margins. 



Ribs. — The ribs throughout are single-headed, and of the usual 

 plesiosaurian type, though proportionally short in the thoracic region. 

 The axial rib is flattened spatulate at the distal extremity; massive at 

 the proximal. The lower surface is nearly flat, the upper surface 

 deeply concave, the anterior border gently, the posterior border deeply 

 concave. The rib of the third vertebra is more expanded distally, 

 with the distal posterior margin somewhat more produced, and the 

 distal border thinned. The fourth rib is more expanded distally, with 

 a more pronounced distal expansion, and the distal border seems to 

 be thickened for cartilage. The next four or five ribs differ only 

 slightly from the preceding ones. The rib of the tenth vertebrae is 

 more elongated and slender. The attachment to the centrum is 

 sessile or nearly so, the head inserted in a shallow pit. In the 

 eleventh vertebra there is a distinct exogenous process, standing out 

 twenty or twenty-five millimeters from the surface of the centrum to 

 the end of which is attached the rib. This process is equally as 

 prominent, or more prominent on the next two centra, the twelfth 

 and thirteenth. The rib of the twelfth vertebra is intermediate in 



