468 BAUR. [Vot. III. 
old animal supraoccipital, exoccipitals, paroccipitals, petrosals, 
are united, but in the young all these elements are free. There 
is much cartilage between the supraoccipital and the petrosal 
and paroccipital. The paroccipital is united to the exoccipital 
by suture; the elements in question.of a young Sphenodon} 
resemble those in Chelone and especially in /chthyosaurus. In 
all other Monocondylia the paroccipital is united very early to 
the exoccipital, forming the paroccipital process of this element. 
In all, the stapes is placed between the paroccipital and petrosal, 
and becomes more and more reduced in size. In Sphenodon 
it is of relatively large proportions. 
In Didelphys the paroccipital is free from the exoccipital, but 
united with the petrosal. This complex, for which the name 
“‘periotic”’ has been introduced, is suturally united with the 
squamosal, in which the quadrate and quadratojugal is also con- 
tained. The stapes is between paroccipital and petrosal and 
very small. 
Now let us consider for a moment the supraoccipital. Ac- 
cording to Professor Huxley this bone in the Monocondylia 
contains on each side an element which is said to be free in the 
embryo. In all the Monocondylia (except the Theromora, of 
which nothing is known as to the point at issue) the supraoc- 
cipital receives on each side the upper semicircular canals to a 
smaller or greater degree; this we find in the lowly organized 
Ichthyosaurus and in the highly specialized Bird. But these 
canals are not lodged in a peculiar element confluent with the 
supraoccipital on each side, called efzotzc by Professor Huxley, 
but in the swpraoccipital itself.. The part of the supraoccipital 
containing the canals never develops from a distinct centre of 
ossification. There is no indication of it in the /chthyosaurus 
and young Sphenodon, in which the supraoccipital is very small. 
I thought to find this element in the Testudinata, in which the 
paroccipital remains always free, but in embryos of Cheloniide, 
Chelydridz, Trionychidz, Emydide, I failed to do so. 
In Mammals the upper semicircular canals do not reach into 
the supraoccipital. 
As result of the examination of the skulls of the higher forms 
1 I am very much obliged to Prof. B. G. Wilder, of Ithaca, for a young specimen 
of Sphenodon \ent for examination. This animal forms in a half-adult stage a won- 
derful specimen for a basis of osteological studies, 
