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DEVELOPMENT OF THE SKULL IN THE MAMMALIA. 197 
in the Prototherian direction ; if carried further, and the bone were flattened out, 
we should have an incus very similar to that of a Monotreme. 
The long crus is hollowed out into a pneumatic boat, its long opening looks forwards 
and outwards (fig. 9). 
The stapes of the adult (Plate 25, fig. 10) is greatly altered from that of the larger 
young (Plate 28, fig. 11), at the time when it is fastened in the fenestra ovalis by the 
bony arterial tube. 
Then it had narrow sides, and its incudal condyle had a rounder shape ; now, that 
articular face is oblong, and the sides have become inflated, as also the base ; so that 
the whole bone is a sort of lagena, with an oblong apex and a bulbous base ; whilst the 
flattened sides are perforated, one by a large, and the other by a small, oval fenestra, 
The sides of the base are neatly limbate, the rim being adapted to the edges of the 
fenestra ovalis. That there is nothing in the structure of the skull of the adult Mole 
that is not in harmony with the life and habits of the creature, I feel certain; but 
questions of that sort are of less interest than the correspondences seen in this 
terricolous Mammal with that we are familiar with in Reptiles (Crocodilia) and Birds. 
Besides the general pneumaticity of the whole hind skull, we have the hollow counter- 
parts of the quadratum, articulare, and columella of those high Sauropsida. 
On the skull of the Common Shrew (Sorex vulgaris). 
There are three native species of the Soricidee—Sorex vulgaris, S. pygmeus, and 
Crossopus fodiens. I have worked out the skull in the first of these, comparing it in 
its adult condition with that of the last, or largest, kind. 
My materials yielded me four stages, as follows :— 
Ist Stage.—Embryos, § ripe; 84 lines long from snout to root of tail (Plate 16, 
figs. 10,°11). 
2nd Stage.— Ripe embryos ; 94 lines long. 
3rd Stage—Young from the nest, 10 or 12 days old; 14 inch long (Plate 16, fig. 12). 
4th Stage. —Adult Shrews. 
Besides these I obtained early embryos, with the somatomes well formed, a little 
less developed than my earliest embryo of the Mole (Plate 16, tig. 1); these were merely 
examined externally. 
The First Stage here given was examined in sections, but as these corresponded 
very literally with those of the younger Mole, already described, they were not 
figured, In these almost ripe young the general form is such as one might imagine in 
some early Tertiary types, in which the characters of the Orders of Mammalia now 
existing had not been developed. 
It is a simple pentadactyle form, very similar to my third stage of the Mole 
(Plate 16, fig. 4), but having smaller fore limbs that are further back from the head. 
