1885.] 



On the Skull in the Mammalia, 



135 



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 

 swollen basis cranii, all air galleries within, is so excavated that tne 

 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 in the ossi- 

 fication, in the young, three parts grown, of the sheath of the stapedial 

 artery, which for a time holds the stapes in its place. It is, however, 

 absorbed afterwards, but remains in the related genus Myogale. In 

 nearly half-grown young Moles the malleus is quite like that of the 

 Marsupials ; it is an evident " articulare," with copious wild growths 

 of bone, sub-distinct, which answer to the " angulare" and " supra- 

 angulare " of a Reptile or Bird. This malleus in its articular part 

 has two endosteal and one ectosteal bony centre. 



Meckel's cartilage, long continuous with the Malleus, is nearly as 

 massive as in the Hedgehog, and has a more distinct separate ossi- 

 fication in its sub-distal part, a long, independent, but temporary 

 lujpobranchial bone. 



The Mole shows a most remarkable development of the endo- 

 cranium, which, twenty years ago, suggested to me that its skull 

 retained unmistakable Monotrematous characters. In large young of 

 the Echidna and Ornithorhynchus the solidity of the chondrocranium is 

 immense, like that of a Chimceroid Selachian, and the investing bones 

 are thin and splintery. I have not made out the mode of ossification 

 of the inner skull in those types, but in spirit, if not in. the letter, 

 the Mole agrees with them, that is, in the great development and 

 independence of the inner skull. The opisthotic bone ossifies the 

 normal petro-mastoid region, whilst the prootic bony centre begins in 

 its right place on the front edge of the cartilaginous capsule, and 

 then runs away from it into the wall of the skull. Thus there is a 

 large bony tract in the temporal region between the squamosal and 

 the large interparietal, which is not one of the ordinary ectocranial 

 bones, but an endo-cranial bony tract overshadowing and yet imitating 

 the true temporal bone or squamosal. This bone is represented by 

 three separate centres in osseous fishes, namely, the prootic, pterotic, 

 and sphenotic, whilst their true auditory region is partly ossified by 

 the epiotic aud opisthotic ; the epiotic is only sub-distinct in the 

 Mole. If I am asked why I dive so far down for my illustrations, 

 instead of being satisfied with what Reptiles and Birds would show 

 me, my answer is that these are often of no use for comparison, as 

 they are as thoroughly specialized for their own mode of life as the 

 Mammalia, generally, and are as completely, and often more completely, 

 transformed from the original archaic type or types. Thus the Mole, 



