Vv INTESTINE AND. BRAIN 109 
p- 109) are characteristic of the group, and the latter bone is 
peculiar to the Monotremata among mammals. So, too, is the 
large coracoid. In the scapula there is a spine which coincides 
with the anterior border of that bone. The arrangement of the 
muscles in this region proves conclusively that this projection is 
the homologue of the spime and the acromion of other mammals. 
Here, again, we have a point of likeness to the Cetacea! In 
the pelvis the acetabulum is perforate (in Kehidna), as in 
Sauropsida. 
Considering the numerous very 
archaic features which the general 
structure of this group displays, 
it 1s surprising to find how typi- 
cally mammalian they are in 
certain other peculiarities. The 
mammalian diaphragm, one of 
the distinguishing features of the 
class, 1s perfectly normal in the 
Monotremata. The alimentary 
canal shows no great divergences 
from the normal structure. The 
stomach is almost elobular, with Fie. 52.—Side view of right half of the 
4 projecting pyloric region in shoulder girdle of a young Echidna 
: (Achidna aculeata). x1. a, Acro- 
Ornithorhynchus ; the intestine 
is divided into a “small” and 
“large” intestine by a_ slender 
eaecum. The liver has the sub- 
divisions that this organ usually 
mion ; ¢, coracoid ; cb, coracoid border ; 
cl, clavicle; css, coraco-scapular 
suture ; ec, epicoracoid ; gb, glenoid 
border ; gc, glenoid cavity ; ic, inter- 
clavicle ; pf, postscapular fossa ; ps, 
presternum ; s, spine ; ss, suprascapu- 
lar epiphysis ; ssf, subscapular fossa. 
i : (From Flower’s Osteoloqy. ) 
shows in the Mammalia. How- 
ever, the presence of the ventral mesentery and of the 
abdominal vein in Hehidna and Grnithorhynchus has already 
been mentioned as a distinctive character. The peculiar and 
apparently partly primitive valve of the right ventricle has been 
described above (see p. 66). The brain is in most respects 
mammalian in its characters, but naturally shows some important 
differences. ~ Dr. Elliot Smith, who has most recently studied 
this question,” is of opinion that the size of the cerebral hemi- 
spheres is not at all reptilian ; indeed, it “ greatly exceeds that of 
? Muscular insertions and attachments do not, however, altogether support the 
comparison, 2 Journ. Anat. Phys. 1899, p. 309. 
