22 
MONOTREMATA. 
longitudinal groove into four parts, called corpora qitadri- 
gemina; in the Monotremata, the same mass is divided in 
the transverse direction; the anterior portion is also indis¬ 
tinctly divided by a longitudinal groove, but the hinder por¬ 
tion, wliich is much smaller, is undivided, and thus forms a 
single transverse band. 
In the skeleton, one of the most striking peculiarities, as 
compared with other mammalia, is displayed in the structure 
of the chest and blade-bones: these together (in the Omi* 
thorliynchus), form a ring encircling the fore part of the 
body; the upper part of this ling is formed by die too 
scapulae, or blade-bones, and the lore, or under part, by the 
chest-bones. These latter, sternal, bones, are live in number: 
the foremost (the episternal) is a broad flat hone, considera¬ 
bly expanded at the lower extremity, and tlirows out. nearly 
at right angles, a long slender branch on eacli side at the 
anterior extremity. The second (the manubrium stemi) is 
also a broad flat hone, and of a triangular form; the three 
following, posterior, hones are very narrow. We have next 
to notice the clavicles, which are here long and slender hones, 
nearly meeting in the mesial line of the chest, nnd os they 
are extended outwards to the shoulder, are joined to, and run 
parallel with, the upper part of the T-shaped episternal bone. 
Lastly, in the chest, are lour other bones; the two coracoid, 
and the two epicoracoid bones: the former are joined to the 
scapular hones, and as they run downwards, converge' and 
meet the lower extremity of the episternal bone. The epico- 
racoids are flat, broad bones, which nearly occupy the whole 
of the interspace between the coracoids and the epistemum; 
they partially overlap the inner side of the latter bone 1 . In 
1 In the above description of the shoulder and chest of the Omitliorhynchus, 
the author 1ms followed Professor Owen's views, as regards the signification of 
the various bones.—See the article Monotremata , in Todd's Cyclopedia of 
Anatomy and Physiology. 
