78 
ACTIN OPIEKXGTI. 
The chondrocranium in Lejpiclotus is well ossified, and the basi 
cranial axis is straight. The basioccipital exhibits on its posterior 
face a ver} T deep conical fossa for the notochord; and the inferior 
aspect of the bone is marked by a broad longitudinal groove, with a 
flattened rim on each margin, probably indicating the backward 
extension of the basicranial canal (for the recti muscles of the eye) 
as far as the occiput. In one example of L. latifrons (no. P. 6841) 
there is also some appearance of one vertebral ring being fused 
with the basioccipital on its hinder face. The exoccipital forms a 
considerable part of the lateral wall of the brain-case, and is per¬ 
forated as usual by a large transversely-oval foramen for the exit 
of the vagus nerve. The pro-otic has a still larger extent than the 
exoccipital, and exhibits a d^ep oblique notch at its anterior margin 
for the passage of the fifth nerve. The characters of the inter- 
orbital septum, pterotic and opisthotic bones are still unknown : t 
and of the ethmoids it can only be said that they surround a pair 
of long closed canals for the passage of the olfactory nerves. The 
membrane bones of the cranial roof form a continuous shield, but 
do not extend backwards quite so far as the occipital border. 
There are two principal pairs of bones, the short parietals behind 
and the longer frontals forwards, not bilaterally symmetrical, but 
uniting in all directions by irregularly angulated sutures, which 
vary in different species. More anteriorly there would be nasals, 
but these are not satisfactorily known. The long and narrow 
squamosal element on each side reaches as far forwards as the 
anterior extremity of the parietals, but not so far backwards as the 
hinder margin of these bones. It is considerably overlapped by 
the supratemporal series of plates, and on the inner aspect there is 
an undetermined amount of connection with some of the ossified 
otic elements. Its surface for articulation with the upper end of 
the hyomandibular is clear, and a triangular walled area at the 
hinder end of its inner surface denotes the boundaries of the small 
temporal fossa. Postfrontal and prefrontal membrane bones are 
not differentiated from the circumorbital ring, and the cheek-plates 
are irregularly subdivided, being often different even on the two 
sides of the same head. The general plan of the cheek-plates, 
however, is distinct, there being a complete circumorbital ring, a 
semicircle of suborbitals, and a short deep series of preorbitals 
flanking the ethmoid region. The maxilla is a very delicate bone, 
deepest behind, tapering forwards, and terminating in front in an 
inwardly-directed process for articulation with the palatine; its 
oral margin is provided with a series of styliform teeth. The 
premaxilla is a smaller, stouter bone, also with a single series of 
styliform teeth, aud bearing at its inner extremity a very large 
