XIII 



PHYLUM CHORDATA 



515 



consisting of two narrow bars with a wide space between them. 

 The epiphyses are very distinct discs which often remain separate 

 from the bodies up to a late period. The neural spines are well 

 developed. The zygapophyses are not well developed, and are 



uln 



rad 



acr 



FIG. 1155. Shoulder-girdle of Three- 

 toed Sloth (Braaypus tridactylus). 



acr. acromion ; el. clavicle ; cor. 

 coracoid. 



7TI.C.1 



FIG. 1156. Right manus of Three-toed 

 Sloth, cun. cuneiform; lun. lunar; m.c.l, 

 first metacarpal; m.c.5, rudiment of fifth 

 metacarpal ; pis. pisiform ; rod. radius ; sc. 

 scaphoid ; trd. m. trapezoid and magnun; 

 united ; uln. ulna ; unc. unciform. 



absent in the posterior portion of the trunk. In the absence of 

 hind limbs there is no sacral region. The caudal region consists 

 of numerous vertebrae beneath which, opposite the intervertebral 

 spaces, are a series of chevron bones (chev.). 



In the Whalebone Whales only 

 one pair of ribs articulates with 

 the sternum, and none articulate 

 with the bodies of the vertebra?, 

 but only with the transverse 

 processes. In the Toothed 

 Whales only a small number are 

 connected with the sternum, 

 sometimes through the inter- 

 vention of intermediate ribs, 

 and a few of the anterior only, 

 in most cases, articulate with 

 the bodies of the vertebrae ; but 

 in some a greater number ar- 

 ticulate with both transverse 

 processes and bodies by distinct 

 tubercles and heads. 



The sternum varies in shape. 

 Sometimes it consists of a pre- 



sternum and a series of several sternebrse without xiphisternum ; 

 sometimes (Fig. 1161) it is a continuous plate of bone, occasionally 

 with median notches or fontanelles. 



In the skull (Fig. 1162) the brain-case is rounded, the jaws 

 greatly elongated, often unsymmetrical, The parietals (Pa) do 



Fid. 1157. Pes of Three-toed Sloth. 



ast. astragalus ; calc. calcaneum ; cbd. 

 cuboid ; fb. fibula ; mesoc. mesocuneiform ; 

 metat.l, vestige of first metatarsal; metat.H, 

 vestige of fifth metatarsal ; nap. navicular ; 

 lib. tibia ; x, peg-like process at distal end 

 of fibula. 



