498 ZOOLOGY SECT. 



sometimes a mere ring of bone. In other cases it not only gives 

 rise to a tube for the external auditory meatus, but also forms the 

 bulla tympani, a dilated bony process containing a cavity. 



The occipital region presents two condyles for the articulation of 

 the atlas. 



The mandible consists in the adult of one bone, the equivalent 

 of the dentary of Sauropsida, on each side the two rami, as they 

 are called, being in most Mammals closely united at the symphysis. 

 The mandible articulates with an articular surface formed for it by 

 the squamosal bone, below the posterior root of the zygomatic arch. 



The hyoid consists of a body and two pairs of cornua anterior 

 and posterior ; of these the anterior pair are usually longer, and 

 consist of several bones, the most important and most constant of 

 which is the stylo-hyal, connected usually with the periotic region 

 of the skull. The posterior cornua or thyro-hyals are usually much 

 smaller. 



The ratio borne by the capacity of the cranial cavity to the extent 

 of the facial region varies greatly in the different orders. The 

 greater development of the cerebral hemispheres in the higher 

 groups necessitates a greater development of the corresponding 

 cerebral fossa of the cranium. This is brought about by the 

 bulging upwards, forwards, and backwards of the cranial roof, 

 resulting in a great modification in the primitive relations of certain 

 of the great planes and axes of the skull (Fig. 1138). Taking as a 

 fixed base line the basi-cranial axis an imaginary median line 

 running through the basi-occipital, basi-sphenoid, and pre-sphenoid 

 bones we find that the great expansion of the cerebral fossa in the 

 higher Mammals leads to a marked alteration in the relations to 

 this axis (1) of the occipital plane or plane of the foramen magnum ; 

 (2) of the tentorial plane or plane of the tentorium cerebelli (a 

 transverse fold of the dura mater between the cerebral hemispheres 

 and the cerebellum) ; and (3) of the ethmoidal plane or plane of the 

 cribriform plate of the ethmoid. In the lower Mammals (A) these 

 are nearly at right angles to the basi-cranial axis. In the higher 

 groups, by the bulging forwards and backwards of the cranial roof, 

 the occipital and tentorial planes incline backwards and the 

 ethmoidal forwards, until all three may become approximately 

 horizontal. At the same time there is produced a change in the 

 relations of the basi-cranial axis to the basi-facial axis a line passing 

 along the axis of the face between the mesethmoid and the vomer. 

 In the lower forms the angle at which the basi-facial axis, when 

 produced, meets the basi-cranial is an exceedingly open one ; in 

 the higher forms, owing to the downward inclination of the facial 

 region, this angle decreases in size, though it is never reduced to less 

 than a right angle. 



The pectoral arch of the Theria has fewer distinct elements than 

 that of the Sauropsida. The coracoid, which in the latter is a large 



