222 B. NATURAL HISTORY. 
arch, as in Ichthyosaurus and Lystrosaurus, differing only in its 
elevation above the occipital elements as an arch, instead of being 
closely depressed upon them. It has been already suggested in 
this essay, that this portion may include the epiotic element. 
The second peculiarity is observed in Sphenodon, and is quite 
unparalleled. This is that the opisthotic expands over the exter- 
nal face of the squamosal, concealing it from outside view, and 
occupies the greater part of the posterior face of the parieto-quad- 
rate arch. Its position suggests at first the inquiry whether the 
identification of the two elements here adopted is not the reverse 
of the true one. The relations of the opisthotic to the exoccipital 
are, however, as elsewhere; while the squamosal forms the inner 
side of the zygomatic arch behind, and occupies in part the posi- 
tion seen in Lystrosaurus. 
The third peculiarity already described is the posterior inferior 
production of the squamosal in Ichthyosaurus and Lystrosaurus. 
In the latter it is very remarkable, and covers the outer side of 
the quadrate completely. 
The last feature is alluded to for the purpose of carrying 
the homology of the squamosal into the Batrachia. Huxley 
(Elements Compara- 
tive Anatomy) does 
not allow himself to 
compare any element 
in that class with this 
” and, alluding to the 
“tympanique ” (Cu- 
Fig. 21.— Cranium of Rana mugiens; profile. vier) of the frog, 
says it is too different from the squamosal to be compared with 
it. If, however, he had had the cranial structure of Lystrosaurus, 
he could no longer have 
doubted, but would have 
homologized them at 
once. (Figs. 21, 23, Sq.) 
Dr. W. K. Parker has 
ventured on this step, 
and identified the squa- 
Fig. 22. — Same, with squamosal, prefrontal, and 
mosal in the Batrachia, maloquadratojugal'(MQJ) removed.’ P.S, Parasphe- 
in accordance with the noid; V, Vomer; E, Ethmoid. 
present views, on embryological grounds alone.* 
* See London Philos. Trans., 1865, p. 162. 
