OSTEOLOGY. 297 
the posterior sacral only a slightly modified caudal.’’ He assigns to the Echidna 
the formula, C7, D 16, L 5,8 2,—, Ca10 = 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 lumbars. 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 united 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 belief that the sacrum includes 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 lumbars 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 11. 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 diminishing series. The usual vertebral 
formula for the Proechidna may therefore be considered as C 7, D 16, L 4, 83, 
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 in 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 to12. 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. 5 
Riss.— Six of the ribs articulate directly with the sternum as in the Hchidna. 
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 ealls P. villosissima from two lateral centers. The three succeed- 
ing sternebra are slightly narrower than the posterior end of the manubrium. 
