DEVELOPMENT OF THE SKULL IN THE MAMMALIA. 
273 
Moreover the tympanic region has only one “ annulus/’ the outer bone. There is no 
separate “ os bullae.” Instead of the latter there is a crescentic shell of bone which 
grows from the basisphenoid, greatly increasing the size of the tympanic cavity. In 
the endoskeleton, in front of the tympanic cavity, there is a remarkable ridge of bone 
growing outwards from the alisphenoid. That ridge is the remnant of the alisphenoidal 
tympanic wing of the Marsupial, and a shell of bone growing from the basisphenoid is 
the same morphological element as the separate “ os bullae,” but it has lost its in¬ 
dependence. The higher Mammalian type is fully reached in the thorough freedom 
of the alisphenoid from the general cranial wall. This character, indeed, is intensified 
into the special diagnostic of an Insectivore, for it lies almost wholly outside the 
orbitosphenoid. Here the sphenoidal fissure—which in this case lets out the 1st and 
2nd branches of the 5th, but not the optic, nerve, that nerve having its own foramen in 
the orbitosphenoid,—is not a mere gap, but a side passage, a sort of sphenoidal 
corridor, right and left. 
In these things the Hedgehog is higher than the Marsupials, but in some others it 
is lower, or more archaic. These latter characters, which suggest an uprise from a 
more generalized type than the existing Metatheria, are— -first, the development of a 
considerable rod of solid hyaline cartilage in the pterygoid region, a remnant of the 
pterygo-quadrate of the Ichthyopsida ; and secondly, the presence of a persistent 
pituitary hole, which is connected with a curiously specialized structure, only seen in 
typical Insectivores, namely, a hollowing out of the basis cranii beneath the pituitary 
region. 
A third archaic character, not seen in the existing Marsupials, is the huge relative 
size, long persistence, and separate distal ossification of Meckel’s cartilage, so that in 
the embryo Hedgehog, and even in the Nestling, the primary lower jaw is as large as 
in Fishes, generally, scarcely excepting the Selachians. 
The ossicula auditus are typically Entherian ; we have lost the imperforate stapes or 
columella, the interhyal is very small or absent, and the malleus and incus are 
much like what we find in the higher Mammals generally. The pneumatic!ty of the 
skull is much reduced ; and the olfactory region is almost double the relative size of that 
of a Marsupial. In the head of another family of the Insectivores, namely, the Mole 
(Talpa europcea ), there is much that is in accord with what is found in its distant 
relation, the Hedgehog, but in it there are evident signs of degradation, or of relapse 
into what is Marsupial in character. 
The nasal labyrinth is relatively immense, and the skull walls below, laterally, and 
behind, are as exquisitely pneumatic as in the Flying Marsupial ( Petaurus ), the Bird, 
or the Crocodile. The swmllen basis cranii, all air galleries within, is so excavated that 
the hinder sphenoid, both base and wings, largely helps the flat single tympanic to 
form the drum-cavity. 
The pituitary hole does not exist, but there is a considerable pterygoid cartilage ; 
the ossicula, in the adult, are normal, but a curious special character is seen during 
ossification. In the young the bone grows along the sheath of the stapedial artery, 
MDCCCLXXXV. 2 N 
