xiii MAMMALIA 445 



The cartilaginous cranium of the embryo becomes almost entirely 

 replaced by bone : it is only in the neighbourhood of the external nares 

 that cartilage persists to any considerable extent. The individual bones 

 of the skull remain as a rule separated until late in life by irreguUr 

 sutures. The general form of the skull resembles rather that of the 

 Amphibian than that of the modern Reptile : in particular the cranial 

 cavity extends right forward to the neighbourhood of the olfactory organ 

 whereas in the typical modern reptiles and birds the two orbits -are 

 approximated close to the mesial plane, remaining separated by a thin 

 interorbital septum into which the cranial cavity does not extend. 

 Further, in correlation with the great development of the cerebral 

 hemispheres, the cranial roof bulges upwards and the nasal region of the 

 skull has the appearance of being rotated downwards in relation to the 

 cranial cavity. 



In Echidna and in the embryos of ordinary mammals the skull articu- 

 lates with the first vertebra by a single transversely elongated joint surface 

 (occipital condyle) but in the typical mammal the central part of this 

 disappears so that in the adult there are two separate condyles one on 

 each side. 



A striking characteristic of the mammalian skull is afforded by 

 the apparent absence of the quadrate — the large and conspicuous bone 

 with which the lower jaw articulates in reptiles and birds. Its dis- 

 appearance is associated with a remarkable change that has taken place 

 in the auditory region. The columella — the bony strut which in the 

 amphibians and reptiles transmits the vibrations of the tympanic mem- 

 brane across the tympanic cavity to the contents of the auditory capsule 

 — is replaced functionally in the mammal by a chain of three separate 

 bones — malleus, incus and stapes. It is commonly held by vertebrate 

 morphologists, on the ground mainly of their method of development in 

 the embryo, that this chain of ossicles has not arisen in evolution as we 

 should expect by a simple process of segmentation of the originally 

 continuous columella but by the absorption into the tympanic cavity 

 of two bones originally outside it, namely the quadrate (incus) and a 

 small bone at the cranial end of the lower jaw in the reptiles — the 

 articular (malleus). If this extraordinary theory is correct, and the 

 great majority of morphologists accept it, it necessarily follows that the 

 lower jaw has in the course of mammalian evolution developed a new 

 articulation with the skull to replace its original articulation by way of 

 the quadrate. 



Another characteristic feature of the mammalian skull indicative of 

 a high degree of adaptation to a warm-blooded terrestrial existence 



