560 CHORDATA. 
and most specialised of the Armadillos, known as the Glyptodonts, are 
extinct forms found in the Pleistocene. The body was enveloped in 
one huge shield into which the head could be retracted. The vertebral 
column is ankylosed together, the shield preventing free movement 
(cf. tortoise). 
From this brief description of the Xexarthra, it will be 
seen that the present forms are only the remains of an 
extensive group of mammals which once held a dominant 
position in the Neogzan realm. Why such powerful 
creatures as Megatherium and Glyptodon have disappeared 
is a question that has puzzled many. All we can say is 
that a type, like an individual, has a limited part to play on 
the stage of organic evolution, determined by the relation- 
ship of an organism to its environment. 
SUB-ORDER II.—NOMARTHRA. 
This small sub-order contains two families which are 
doubtfully related to each other. They are terrestrial or 
arboreal and feed on ‘‘ants” or termites. Hence in them 
is found the same elongated snout, small mouth, long 
mobile tongue and large salivary glands, as in the ant- 
eaters and Echidna. The uterus is bicornuate, or there 
are two uteri, and the placenta is non-deciduate and diffuse 
(or zonary, modified from the diffuse). There are no extra 
articular processes on the vertebree. 
Family I.—Orycteropodidz.—Aard-varks or earth pigs. The 
aard-vark of South Africa is a nocturnal and partially fossorial animal. 
Its body is sparsely covered with hairs. It is plantigrade with four and 
five toes all armed with strong claws. The teeth are unique in structure 
amongst mammals. They grow from persistent pulps, gradually pushing 
forward in a manner similar to that found in the elephant and the 
kangaroo. There are usually five on each side in use at the same time 
and about ten in all. All but the three last are preceded by a milk set, 
which are absorbed before cutting the gum. This appears to indicate 
premolars and molars and a possible degeneration from a higher type 
of heterodont dentition. 
Family II.—Manidz or Pangolins.—The pangolins are elongated, 
terrestrial, fossorial animals: many can climb trees. They have the 
body clothed in a series of large overlapping scales of horny epidermic 
origin. On the under-surface there is usually hair only. The tail is long 
and protected in a similar manner to the body. Like the Armadillos, 
they can usually roll themselves into a ball. The skull, especially in the 
jaw region, is modified for the ‘‘termite-eating” habit, as in the anteaters: 
