ZOOLOGY. 197 
in this, as in other determinations of the same author, is the squa- 
mosal.* sd 
In the same way Giinther, in describing Sphenodon (Philos. 
Trans., 1867), calls the quadratojugal arch the zygomatic, and the 
zygomatic the “temporal arch,” employing a new name to desig- 
nate it. Stannius (Zootomie der Amphibien) appears to have cor- 
rectly identified the zygomatic arch in Lacertilia, but erroneously 
in the Crocodilia. 
Before proceeding to determine more exactly the homologies of 
the posterior cranial bones, I will describe the cranial structures of 
Ichthyosaurus and Lystrosaurus, as our literature appears as yet to 
be deficient in these points. 
2. On the Cranium of the Ichthyopterygia. 
Commencing with the foramen magnum and occipital condyle, 
as fixed points, the connections of the bones, as they succeed each 
other forwards, may be safely considered. 
All four of the occipital elements contribute to the margin of 
the foramen magnum, the supraoccipital not being excluded as in 
Crocodilia, Anomodontia, etc. The external or lateral margin of 
both exoccipitals and basioccipital are excavated by a large foramen. 
The continuous margin of both between these points is united to 
a bone which extends outwards and upwards, and which con- 
tributes by its superior and inferior margins to the outlines of the 
foramina just mentioned. Exterior to these, from the basioccipi- 
tal to near the apex of the supraoccipital, there are no bones 
suturally united, and there is a vacuity in this positién not seen in 
any other Reptilian cranium. 
From the exterior margin of the inferior foramen, a subcylindric 
bone extends outwards. It is contracted medially, and is not in 
sutural connection with any other. Immediately exterior to it is a 
flat subvertical bone, which, as it bears the articular condyle for 
the mandible on its lower extremity, is no doubt the quadrate. 
That it is such is also proven by the fact that it is anteriorly con- 
nected to the malar bone by a quadratojugal. 
If we now turn to the lateral view of the skull, we observe the 
zygomatic arch, as determined above; 7. ¢., the superior of the two 
extending from the malar, and that which supports the postorbital 
arch. The bone which forms its posterior half must be the squa- 
* This description, by the way, differs from Von Meyer’s figures of Nothosau- 
rus, where but one arch is represented. 
