714 STUDIES FOR STUDENTS 
poral arch. The teeth were sharp and strong, conical in form 
and often recurved; they were set in separate sockets or alveoli. 
The pectoral and the pelvic girdles in the most advanced of the 
group were greatly expanded to serve as a protective shield for 
the thoracic and the abdominal regions. 
The Plestosaurs descended from land-living animals with well 
developed limbs just as the /cthyosaurs did. In the earliest of 
the Plestosaurs the fore and hind legs were those of an animal 
that walked on the dry land at least as well as the modern 
alligators, but in the later and more specialized forms, the limbs 
are reduced to paddles which were useless for any purpose but 
that of swimming. The general appearance of the brute has 
been aptly described as that of a ‘turtle strung on a snake.” 
There are generally recognized three families of the Pleszo- 
SQUrs: 
Nothosauride. 
Plesiosauride. 
Pistosauride. 
Nothosauride.—These were small forms that were most nearly 
related to the parent form of the whole group. The limbs were 
more or less well adapted to progression on land and there were 
five functional digits on each foot. 
Nothosaurus, from the Triassic (Muschelkalk) of Byreuth and 
from other parts of Germany and from the Tyrol and the Keup- 
fer Of St..Cassian in) Erance, was. one of ithe small forms seeke 
was about nine feet long. The skull was long and flat. The 
orbits were small and were located in the middle portion 
of the skull. The temporal fossa was quite large, taking up the 
posterior third of the skull. 
The anterior nares were small and located far forward but not 
at the extremity of the rostrum. The teeth were numerous, 
smail, and of nearly equal size in the posterior portion of the jaws, 
they formed large recurved tusks in the fore part, and were set 
far apart. There were 20 cervical vertebre, 25-30 dorsals, and 
about the same number of caudals. 
Closely related forms from the same horizon, that are known 
