37 
The triceps seems to be a strongly developed muscle in all Mar- 
supials. 
ln Thylacinus the origin of the scapular head is extensive, according 
to Cunningham,* reaching along the whoie length of the posterior 
margin of the scapula. In most other cases, however, the scapular 
origin does not occupy more than a third or a half of that border, and 
there is in no case an extension of the muscle upon the outer surface 
of the bone such as is correlated in Notoryctes with the development 
of the post-scapular spine. 
The outer and inner humeral heads are stated by Macalister+ to be 
inseparable from each other in Marsupials, but this is by no means 
universalt, nor is it so in Notoryctes. 
The triceps in Monotremes is an exceedingly extensive and powerful 
muscle, especially in Echidna. The scapular part arises in the latter 
from a well-marked crest on the outer surface of the scapula, which 
extends dorsally from the lower (ventral) end of the scapula to near 
the vertebral border. This tricipital crest has been regarded by 
Flower and Gadow as the morphological post-scapular border, and 
this view has been elsewhere discussed by Dr. W. J. S. McKay and 
the writer.§ We cannot confirm Meckel’s description of a segmentation 
of the scapular triceps into three parts|| in Ornithorhynchus. 
The outer humeral head in Hchidna is probably represented merely 
by a small tendinous slip arising external to the origin of the supinator 
longus from the base of the radial tuberosity. It is separated from 
the very large inner humeral head by the musculo-spiral nerve. The 
inner head occupies the entire posterior surface of the shaft of the 
humerus (Mivart‘]). 
It is amongst the Edentata that we meet with the closest parallelism 
to the condition of the triceps in Notoryctes. This is owing to the 
very similar development in many members of the order of a second 
or post-scapular spine. A comparison between fig. 8 and Macalister’s 
fig. 24,** representing the corresponding region in Chlamydophorus, is 
rather striking. But in Notoryctes I have not found that the scapular 
triceps is divisible, as in Chlamydophorus, into three portions; while 
in the latter animal again the humeral triceps is one and indivisible, 
and evidently proportionately smaller than in Notoryctes. In Dasypustf 
there are two scapular origins, the largest being from the whole length 
of the post-scapular spine. In Cyclothwrus,{{ where a post-scapular 
spine is present near the actual hinder margin of the scapula, there is 
no division of the scapular triceps into distinct heads. In Bradypus 
also it is undivided. Giebel$$ regards the post-scapular spine (well 
developed, e.g., in Myrmecophaga tetradactyla) as the true morpho- 
logical posterior border (post-scapular) of the scapula. Its develop- 
ment in the form of a second spine seems correlated with a very 
considerable development of the scapular triceps. || || 
Certain Rodents exhibit an approach towards a post-scapular ridge, 
i.e., a marked exaggeration of the outer or dorsal lip of the “ axillary” 
Sixiv. || xxxvii., page 27. ‘Il xxxix., page $86. **xxvii., Pl. xv., fig. 24. 
tt xiv., page 538. ttxxvii., page 248, and xlvii., Pl. 22, fig. 18. §§ xviii., 
page 408. ||| Cf., lxiv. TT xviii., Pl. 71, fig. 11, and xlvii., Pl. xxiv., fig. 14. 
