178 Prof. Owen on the Class Mammalia. 
the young in all this ‘implacental’ division being brought forth 
prematurely, as compared with the rest of the class. 
This first and lowest primary group, or subclass, of Mammalia 
may be termed, from its cerebral character, L YENCEPHALA,*— 
signifying the comparatively loose or discgnnected state of the 
cerebral hemispheres. ‘I'he size of these hemispheres (fig. 1, 4) 
is such that they leave exposed the olfactory ganglions (a), the 
cerebellum (c), and more or less of the optic lobes (B); their 
surface is generally smooth; anfractuosities, when present, are 
few and simple. 
2.—Brain of Beaver. 
1—Brain of Opossum. 
The next well-marked stage in the development of the her he 
is where the corpus callosum (indicated in fig. 2 by the dot ie | 
lines d, d) is present, but connects cerebral hemispheres as ae 
i balk or outward character as in the preceding wt 
class; the cerebruin (a) leaving both the olfactory lobes See 
Sood 
ree 
Tn this subclass the testes are either permanently or tempora- 
rily concealed in the abdomen: there is a common external ge 
hito-urinary aperture in most; two precaval veins (' 
“anterior vense cave’) terntinate in the right auricle. 146 
mosal in most, and the tympanic in many, retain their pr 
* Liv, to loose, and ¢yxéqados, brain, 
+ Ar@ads, smooth, and éyéxqados, brain. 
