xiii PHYLUM CHORDATA 483 



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 tento- 

 rial 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 lasi-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 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, is sometimes 

 recognisable as a distinct element up to a late period (many 

 Marsupials, Sloths), appears to correspond to the bone called 

 epicoracoid in the Prototheria (vide infra). 



In the scapula a spine is nearly always developed, and usually 

 ends in a freely-projecting acromion process. A clavicle is well 

 developed in many Mammals, but is incomplete or absent in 

 others. In the embryo there is, in the position of the clavicular 

 bar, a bar of cartilage, which coalesces with its fellow in the 

 middle line. The cartilaginous tract thus formed segments into 

 five portions a median, which coalesces with the pre-sternum, two 

 small inner lateral, which unite with the clavicles, or are converted 

 into the stemo- clavicular ligaments, and two long outer lateral, 

 which give rise to the clavicles. The median and inner lateral 

 portions appear to correspond to the epi-sternum of Reptiles and 

 Prototheria. An additional small cartilage may represent the inner 

 portion of the procoracoid of Amphibia. A piece of cartilage at 

 the outer end of the clavicle proper is sometimes distinguishable 

 -the mesoscapular segment. 



In the carpus there are four proximal bones scaphoid, lunar, 

 cuneiform, and pisiform. The scaphoid corresponds to the radiale 

 of the typical carpus (p. 76) ; the lunar perhaps represents a second 

 centrale that occurs in some Amphibia ; the cuneiform is probably 

 the intermedium, and the pisiform the ulnare. 



The centrale is present sometimes as a distinct ossification : the 



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