204 
MR. W. K. PARKER OK THE STRUCTURE AND 
Between the forks of the main vomer (v.), behind the middle ethmoid (_p.e.) a small, 
thick crescent of bone is seen lying crosswise with its convexity forwards ; this is the 
independent presphenoid (_p.s.), a bone not found in the Hedgehog and Mole.* 
On this face the orbitosphenoids are not seen, or very slightly (see figs. 3, 4, o.s .); 
the basisphenoid ( b.s .) is separated from the presphenoid by a broad tract of cartilage; 
the bony tract is four times as long as that synchondrosis, and, in front, three times as 
wide. Behind, it is twice as wide, and so also is the spheno-occipital dividing 
cartilage; this is also somewhat more extended, axially, than the front tract. Here 
we have the abortive development of the large diagnostic Insectivorous basisphenoid; 
it is very large, has stout shoulders, on which the alisphenoids ( al.s .) rest, but it is not 
specialised for pneumatic purposes. The anterior fissura lacera ends in the rounded 
notch between the forwardly-thrust alisphenoid, and the narrow anterior synchondrosis. 
These parts will be understood better when we come to the clisinvested endocranium 
(figs. 3, 4); so also will the occipital arch and the auditory capsule. 
The side view (Plate 31, fig. 10) is still more intelligible when the divided skull 
is seen from the inside (fig. 11); this also gives us one of the three views of the endo¬ 
cranium, as distinct from its ectocranial investing scale-bones. 
This view of the skull of a Shrew, about the twelfth night after birth, should be 
compared with the counterpart figure of the skull of the ripe embryo (Plate 29, fig. 7); 
thus the decadence of the deep chonclrocranium and the development of the superficial 
elements of the ectocranium will be understood. Measured along the basal line, the 
septal and ethmoidal (continuous) regions of cartilage are five-sevenths the length of the 
whole axial tract from the fore end of the snout to the foramen magnum; at birth 
that fore part was two-thirds the whole length. It is the snout which has lengthened 
most (relatively), as if to form a proboscis; but there is no segmentation of the car¬ 
tilage enclosing the double tube. The basal line of the snout is strongly, that of 
the septo-ethmoidal region feebly, arched ; nearly all the thick basal part is due to the 
development of that azygous median prepituitary (= pro-chordal) rod, the inter¬ 
trabecula ; for the trabeculae and cornua trabeculae run but a short distance forwards and 
only meet at the mid-line in the region immediately in front of the sella turcica. 
Thus the middle element of the cranial fore-growths is as large as in the Selachian 
Fishes and the embryo Bird. The foremost part of the septum (s.n.) is perforate—a 
very common thing in Mammals, the alee nasi throwing themselves out from the middle 
wall, in forming the valvular nostrils. Up to the anterior palatine foramina this wall 
is very low ; it then gently rises up to the top of the rhinencephalic fossae, with the 
cribriform plate ( cr.p .) as its floor; there is no crista galli projecting from the obtuse 
angle from which the great partition descends, twice as rapidly as it ascended, until it 
reaches the presphenoidal bony centre ( p.s .). 
The fan-shaped upper part of the great orbitosphenoid of the embryo (Plate 29, 
* We stall see this again in ffliynchocyon; and it is also present in some of the lesser Rodents (at any 
rate in Arvicola, Mus, Ac.). Nevertheless, it is a Marsupial character. 
