xin 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 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 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 sterno- 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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