144 
ME. W. K. PARKER ON THE STRUCTURE AND 
The inferior turbinals ( i.tb.) arise from its inner face, and can he seen in the space 
right and left of the vomer. Where the vomer forks, there each fork supports the 
inturned nasal wall, now the floor ( n.f. ), which is finished by the vomer. In the front 
of this part the inturned walls give off the “ precurrent cartilages,” already de¬ 
scribed. At present, the hinder region of the nasal labyrinth forms merely part of a 
scroll, and has not yet closed in to finish the hinder recess. The very solid basal beam 
is seen between the forks of the vomer, first as perpendicular ethmoid (p.e.), and then 
as presphenoid ( p.s .). The proximal part of the orbitosphenoid is hidden by the 
recesses of the nasal labyrinth, the bony centre (0.5.), can, however, be seen in its 
upper part. 
On a lower plane, the large alisphenoids ( cd.s.), with their oblique sinuous outer 
margin, are seen now to be largely ossified ; these bony plates are perforated by the 
3rd branch of the 5th nerve (V 3 .), for they have a large foramen ovale a little behind 
their middle. Between these wings and the basisphenoid ( b.s .) there are still two 
remarkable tracts of cartilage, one of these is the large synchondrosis between the 
alse and the base, and the other is a button-shaped projection ( e.pg .), between the 
pterygoid ( pg ., pg.c .) and the foramen ovale (V 3 .), but a little in front of both. 
This projection is the cartilaginous rudiment of the external pterygoid plate, which 
in this broad-floored skull is in its normally Mammalian position, namely, a good 
distance outside the correlated pterygoid, with its independent cartilaginous nucleus.* 
In this type, and in many of its Insectivorous congeners, the basisphenoidal bony 
cenire (b.s.) runs behind, largely,for some distance, into the alisphenoidal cartilage (al.s.); 
this is a most important diagnostic of a true, normal, Insectivore. It is, however, a very 
gentle modification of that which is diagnostic of the skull of the Marsupials, namely, a 
“tympanic wing,” which grows backward from the alisphenoid. Here, the rudimentary 
tympanic wing arises from that part of the alisphenoid which is ossified vicariously 
from the basal centre ; thus the further growth of the tympanic wing is merely a shell¬ 
like flange of the basisphenoid. Hence the bone which develops round the tympanic 
air-cell in the typical Insectivora, assisting the superadded. annulus tympanicus, is 
basisphenoidal. The alisphenoidal centre, in Marsupials, which forms the front part 
of their drum-cavity, is supplemented by a large, crescentic “ os bullae,” which 
ultimately becomes ankylosed, in most cases, to the alisphenoid. Moreover, the 
extensive pneumaticity of the basis cranii of the Insectivora, at this part, is very 
Sauropsidan, and these types have also a considerable upper tympanic recess inside 
the squamosal bone, as in Crocodiles, Birds, Marsupials, and Edentates. At present 
the basisphenoid (Plate 19, fig. 1, b.s.) does not reach so far forwards as the ali- 
* In the typical Ruminants, and still better, in the genus Cavia among the Rodents, this sphenoidal 
outgrowth is seen to be manifestly the homologue of the “ basipterygoid process ” of the Sauropsida, 
growing as it does from the side of the basal beam. Its visceral correlate, the pterygoid cartilage, which, 
indeed, dominates it, has undergone most remarkable structural modifications in the types in which it 
re-appears, e.g., in Chelonia, Crocodilia, Passerine Birds, and, lastly, in the lower Mammalia. 
