OSTEOLOGY. 297 



the posterior sacral only a slightly modified caudal." He assigns to the Echidna 

 the formula, C 7, D 16, L 5, S 2, — , Ca 10 = 40. In the skeletons of the 

 Proechidna studied, there seems no reason to doubt that three is the number of 

 functional sacral vertebrae in each, anterior to which in M. C. Z. 12,414 and 12,415 

 there are four undoubted Imnbars. In M. C. Z. 12,414, an old individual, 

 four vertebrae enter into the fused sacral mass, but the most posterior of these 

 is very evidently a caudal that has become thoroughly imited by its centrum and 

 articular processes to the last sacral, and by its transverse processes to the pos- 

 terodorsal margin of the ilium. It is therefore reckoned as a caudal. Evidence 

 for the beUef that the sacrum mcludes morphologically but two vertebrae is 

 found in the fact that but two and these the anterior two sacral nerves enter 

 into the lumbosacral plexus. Gervais considered four and three the numbers 

 respectively of lurabars and sacrals in his specimen, so that these may be con- 

 sidered the normal number of each. In the skeleton belonging to the United 

 States national museum, however, there are five lumbars in addition to the 

 three sacrals, evidently an unusual condition. The caudals, as in the Echidna 

 seem usually to be 12 in number, though one (M. C. Z. 12,415) had but 1 1 . There 

 are six chevron bones, the first of which comes between the third and fourth 

 caudals. They are simply small oval ossicles, of which the second is the largest 

 and from it the remaining four form a dmiinishing series. The usual vertebral 

 formula for the Proechidna may therefore be considered as C 7, D 16, L 4, S 3, 

 Ca 12 = 42, which differs from that of the Echidna in having four instead of 

 usually three lumbars, although even this difference may disappear, since four 

 lumbars are said to occur m the Echidna in certain instances. Thus McKay 

 (1894) has tabulated the vertebrae of eighteen specimens of the Australian 

 Echidna and shows that the range of variation is considerable, namely, cervicals 7, 

 dorsals 14 to 17, lumbars 2 to 4, sacrals 3 to 4, caudals 10 to 12. It thus appears 

 that no generic difference in vertebral formulae exists, as once supposed, between 

 the Echidna and the Proechidna for the range of individual variation bridges over 

 the gaps. 



Ribs. — Six of the ribs articulate directly with the sternum as in the Echidna. 

 The sternum in a nearly grown individual, consists of four rib-bearing portions. 

 The first is the manubrium, broadly expanded at its anterior end, where the first 

 pair of ribs meets it, and contracted posteriorly, where it is joined by the second 

 pair. Dubois (1884) figures this first segment as ossifying in a youngish speci- 

 men of what he calls P. villosissima from two lateral centers. The three succeed- 

 ing sternebra are slightly narrower than the posterior end of the manul^rium. 



