506 CHORDATA. 
CHAPTER XXVIII. 
MAMMA LIA—( Continued. ) 
Sub-Class III.— Eutheria. 
TYPES 2 AND 3, HORSE AND OX; 4 AND 55 DOG AND CAT. 
The Zu¢heria mark the culminating point in mammalian 
structure and, as might be supposed, the members of this 
sub-class show the greatest diversity of adaptive modifica- 
tions. Asa general rule we may say that the hemal form 
of embryonic nutrition is highly developed, the chief organ 
forming the heemal placenta being the allantois. The yolk- 
sac placenta, when formed at all, is merely a transitory 
structure of little functional significance. Further advances 
upon the metatherian type are found in the reproductive 
organs. The urogenital and anal openings are, as a rule, 
quite distinct, the perineum separating the two orifices: 
this is especially evident in the male. The lowest part of 
the oviducts, the vagine, are always fused together and with 
few exceptions the second or uterine portion shows also 
varying grades of fusion, such as the bicornuate and 
bipartite uterus: this fusion of the uteri probably has 
partly to do with a reduction in the fecundity. Correlated 
with the high development of a hzmal gestation there is a 
tendency to a reduction in the period of lacteal gestation, 
though the mammary glands are still well developed and the 
mamme are permanent. 
In the skeleton there are important features. The teeth 
are typically diphyodont and heterodont, and it is usual to 
derive the very numerous modifications from the typical 
dentition of 3443. This typical dentition is, indeed, only 
found in very few types. of which perhaps the pig is the best 
known ; but the assumption of loss of certain teeth in some, 
