512 SI GIDIRS JHOMK. SIQIQIEIN TES 
the upward growth of the lower segment of the vertebra of the 
rachitomous type and the downward growth of the upper seg- 
ment and its union with the two lateral segments. 
Cricotus, the best known form is from the Permian deposits 
of Texas, the same locality as the rachitomous forms. It was 
rather long and crocodilian in aspect and the head was covered 
with the same kind of membrane plates as in the other genera 
but they were devoid of sculpture; the orbits were large and 
placed far forwards. 
Diplovertebron from the Gaskohle of Bohemia is known only 
from fragments of the skull, limb bones and vertebre, but the 
vertebre show the same structure as Cvicotus. 
LABYRINTHODONTI (STEREOSPONDYLI).—These are the very 
large form that appeared at the end of the Paleozoic and reached 
their greatest development during the Triassic. They possessed 
complete biconcave vertebre with a central foramen for the pas- 
sage of the notochord. The most peculiar feature of the group, 
is the arrangement of the outer layer of the teeth; instead of the 
usual smooth layer the enamel is thrown into the most compli- 
cated folds so that a cross section of the tooth has an almost 
dendritic appearance. In other features than the teeth the 
group evinces its advanced position among the amphibians; the 
bones of the skull are all well ossified and the base of the skull 
is composed entirely of bone instead of being partly cartilagi- 
nous as in the preceding groups; there is no trace of the 
grooves on the upper surface of the skull to mark the position 
of the sensory tracts; the thoracic region is completely pro- 
tected by the development of the clavicles and the interclavicle 
into large and strong plates that are tightly joined together; the 
posterior abdominal region is also protected by the expansion of 
the bones of the pelvis though not to the extent reached in the 
anterior girdle; the limb bones are all well ossified and the 
carpus and tarsus were entirely bony; the feet were provided 
with strong claws. ‘ 
Trematosaurus from the Buntersandstein, near Bernberg, in 
Germany, was one of the smaller forms; the skull was about 
