FOSSIL VERTEBRATES — REPTILIA 623 
specimens have been found in the same horizon in Bohemia. 
They were in some respects of the most primitive type; the skull 
retained the two post-orbital arches, the teeth were simple in 
structure and without tubercles or basal band; the vertebrze were 
deeply bi-concave and pierced for the passage of the notochord ; 
intercentra were present, and the sacrum was formed of two or 
three vertebree. Not only were they the most primitive mem- 
bers of their order, but they were among the earliest of the 
land-living vertebrates after the Amphibia, and it is very inter- 
esting to note among their characters certain features that clearly 
foreshadow the culminating point of the order, the mammalia. 
Such characters are the great flattening of the quadrate bone 
and the partial surrounding of this element by the temporal 
bones of the skull, and the beginning of a differentiation of the 
teeth into specialized regions. 
Dimetrodon, from the Permian of Texas, is the best known mem- 
ber of the suborder. There were two post-temporal arches, and 
the bones of the temporal region were all separate. The eyes 
were very large, almost perfectly round, and placed far back in 
the skull. The skull itself was abruptly terminated behind, and 
extended forward as a strong and rather high nose. The upper 
jaw was slightly convex on the alveolar border, and was armed with 
many strong, conical teeth that curved slightly to the rear ; there 
were about three incisors in the jaw, the posterior or outer one 
being much larger than the others. Behind these came a consider- 
able interval, marked by a deep notch at the junction of the pre- 
maxillaries and the maxillaries. Posterior to this came one or 
two very large teeth in the position of canines, followed by a 
long series of small conical, slightly recurved teeth of nearly equal 
size. The alveolar border of the lower jaw was concave to cor- 
respond to the curve of the upper, and there were two or three 
large teeth corresponding in position to the canines of the upper 
jaw. The surface of the palate and the pterygoid bones were 
covered by rows of small teeth. The quadrate was much 
depressed, and only appeared for a very small space on the side 
of the skull, being nearly covered by the adjoining bones. The 
