MAMMALIA, 403 
In our narrative of the history of each group we shall start from below 
and ascend the series which we have just presented, commencing accord- 
ingly with Edentata. In a natural method the extinct groups should 
always precede the living ones ; but as they are generally less known, they 
will be found sometimes to follow the latter, and more specially so when 
doubts are entertained with regard to the family and genera to which they 
may belong. 
Orper I. Epenrara. 
The order of Epenrara is composed of comparatively few animals, differ- 
ing widely from each other, but agreeing in the common characters of 
absence of front teeth or incisors, and the presence of feet that are ungul- 
culated, that is to say, terminated by large claws or nails. The Edentata 
existed in larger proportion during the tertiary epoch than in our days, and 
from considerations derived from paleeontological evidences to be discussed 
hereafter, we suppose that they have lived during the deposition of the 
secondary beds, although no remains have yet been found in those deposits. 
It is also a fact of great importance that during the tertiary period the 
animals of this order attained to a bulk far surpassing that of any living 
representatives. 
The Edentata may be divided into three groups, according to certain 
modifications in their organization, the Monotremata, the Edentata proper, 
and the Tardigrada. 
Group 1. Monotremata. 
The group of Monotremata has received its name from the peculiarity 
of having only one external opening for the seminal fluid, urine, and excre- 
ment, as in birds. They possess the marsupial bones, but have no external 
pouch in which the young pass one part of their embryonic life, as in Mar- 
supialia. The mamme themselves have been long a matter of doubt, as 
well as their mode of generation. According to some travellers these 
animals lay eggs. But if the mamme do not exhibit a projecting nipple, 
still the mammary glands have been shown to exist on the abdomen in the 
form of numerous elongated, sub-cylindrical lobes, converging and opening 
into a small oval areola. The ear has no external concha. The struc- 
ture of the skeleton presents many peculiarities by which this family is 
distinguished from any other. The sternum and shoulder bones join and 
encircle the fore part of the trunk. Their brain wants the corpus 
callosum, and the mass called corpora quadrigemina is imperfectly 
divided. 
This group contains but two families, Ornithorhynchide and Echidnide, 
both of them belonging to Australia. | 
Fam. 1, OrnirHoruyncuip#, includes but one genus, Ornithorhynchus, 
607 
