— 313 — 
that during life they were rather readily movable, a condition which 
would, to some extent, prevent their being broken by contact with 
those opposite, while at the same time not diminishing their effi- 
ciency for the capture of prey. 
As to adaptive significance, the dentition of Mesosaurus is well 
fitted for capturing and killing rapidly moving soft prey such as small 
fishes, aquatic insect larvae, etc. 
Vertebral column. 
As regards the vertebral column, the present material suffices to 
elucidate all structures excepting certain details of the atlas and the 
exact number of the caudals, the latter a point of relatively little 
importance. 
Cervical region, 
The neck is rather long, though slightly shorter than the skull, 
and is exceedingly flexible, as shown by the position of the head 
in fourof the examples studied (PI. II. fig. 4, and PI. IV., fig. 10). 
In Plate II., fig. 2, the neck appears unduly short, owing to the fact 
that the skull is displaced backward, covering the first four vertebrae. 
The cervicals number 11, including the atlas(as in 17. ¢enuidens, 
one less than in Stereosternum tunidum), and all except the atlas 
bear movables ribs. The body of the atlas (intercenlrum) is elongate 
transversely and several specimens show what seem to be atlantal 
neural arches which are separate from each other and from the inter. 
centrum. The axis is shorter than the succeeding vertebrae, and has 
a higher and more expanded neural spine. It is impossible to make 
out the odontoid and I can find no evidence of any free intercentru 
behind the atlas. 
Passing backward from the axis to the eleventh or last cervical, 
the vertebrae increase gradually in all dimensions, but especially in 
the size of the blunt lwansverse process which is developed antero- 
jaterally on the centrum, and which supports the single-headed rib. 
The cervical ribs extend obliquelly backward, and, in one specimen, 
that of the eleventh vertebrae is 14"" in length. In an immature 
example the cervical ribs are truncate at the extremity, as in the 
young Stereosternum figured by Osborn (1903, pp. 484 and 485), and 
were doubtless tipped with cartilage. 
5569 "40 
