DEVELOPMENT OF THE SKULL IN THE MAMMALIA, 
201 
(as.). The narrow stalk of these great leafy growths of cartilage is out of sight; it is 
imperforate. A true Insectivorous diagnostic is seen in the sudden widening of the 
basisphenoidal region ( b.s .), which is fee times as long, and three times as wide as the 
presphenoid, and has an open pituitary space ( py .) in its middle, in which it agrees 
with the Hedgehog and differs from the Mole. Around this hole the bony basi- 
sphenoid (b.s.) is forming ; it is circular at present. The alisphenoids (al.s.) are small 
ear-shaped tracts of no greater extent than the cochleae (chi.), and having an oblique 
position, they look forewarcls as well as outwards, and are quite outside the 
plane of the orbitosphenoids. A round notch is seen between them and the base, in 
front; they are gently sinuous in front and on the outside, whilst, behind, they have a 
deep notch for the 3rd branch of the 5th nerve (V 3 .) near their outer angle. The base is 
clear of all alate growths in its middle part, but, behind, it gives off a small perforated 
crescentic wing which is applied to the fore end of the cochlea (chi.) ; this, however, is 
soon lost, and the tympanic wing, so characteristic of the typical Insectivore, is 
aborted. The spheno-occipital region of the basis cranii is elegantly cut away, 
so to speak, to let in the rounded apex of the cochlea, right and left. The basi- 
occipital centre ( b.o .), nearly reaches the foramen magnum, and is growing fast 
towards the lateral notches ; it is polygonal and reptilian. A superficial tract of 
cartilage and of bone, forming the pterygoid (pg.c.,pg.) is shown attached to the base 
of the skull. The visceral arches are well developed. I have figured the upper part 
from the inner side (Plate 30, fig. 16) of the tympanic region. The malleal end of 
Meckel’s cartilage (mJc.) shows a well formed head, a deep and wide tract beyond 
the articular facet, a very large “posterior angular process” and a short straight 
“internal angular process,” or manubrium (m.ml.). The annulus (a.ty.) is already a 
considerable crescent of bone. The incus (i.) is quite normal and w T ell developed, 
and so is the stapes (st. ), the base of which looks towards the eye in this figure. 
Ossification has begun in the perichondrium of the neck of the malleus (ml.), below ; 
it is a thin ectosteal tract. 
Third Stage .— Young Shrews (Sorex vulgaris) from the nest; 10 or 12 days old ; 
and measuring 1^ inch in length, from snout to root of tail. 
The dissected skull at this instructive stage, in its upper view (Plate 29, fig. l), 
shows a cartilaginous snout (al.n.), nearly as long as the long, and relatively large 
nasals (n.). The nostrils (e.n.) are sublateral, and the snout itself is narrow at first, 
and then widens so as to become ventricose where it passes under the investing bones. 
Those bones are very thin at present, and show the elegant radiating lines of growth. 
The premaxillaries (pa,.)* run well up the face, wedging in between the much larger 
maxillaries (rnx.) and the nasals (n.) ; these two pairs of bones run backwards to about 
the same extent at present, widening backwards. The small, almost Ophidian frontals 
* Forjja. read px. 
2 D 
MDCCCLXXXV. 
