xm PHYLUM CHORDATA 513 



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 stylohyal, connected usually with the periotic region 

 of the skull. The posterior cornua or thyro-hyals are in most cases 

 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 corre- 

 sponding 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 cer- 

 tain of the great planes and axes of the skull (Fig. 1126). Taking 

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

 running through the basioccipital, basisphenoid, and presphenoid 

 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 hemi- 

 spheres 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 back- 

 wards 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 meseth- 

 moid and the vomer. In the lower forms the angle (cranio-facial) 

 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 bone, taking a share at its dorsal end in the bounding 

 of the glenoid cavity, and at its ventral end articulating with the 

 sternum, is never present, in the adult, as a distinct bone. In 

 the young of many Mammals it appears to be represented by a 

 small ossification which enters into the glenoid facet ; but this 

 very soon coalesces with the scapula. The coracoid process which 

 is a separate ossification in the young Mammal, and. though in 

 most instances completely fusing with the scapula and with the 

 smaller coracoid element, is sometimes recognisable as a distinct 

 element up to a late period (many Marsupials, Sloths) appears to 



VOL. II I I 2* 



