282 THE ANATOMY OF YERTEBIIATED ANIMALS. 



birth, simply withdrawn from the uterine fossge, into which 

 they are received, and no part of the maternal substance is 

 thrown oif in the form of decidua, or maternal part of the pla- 

 centa. In the deciduata, on the other hand, the superficial 

 layer of the mucous membrane of the uterus undergoes a 

 special modification, and unites, to a greater or less extent, 

 with the villi developed from the chorion of the foetus ; and, at 

 birth, this decidual and maternal part of the placenta is thrown 

 off along with the foetus, the mucous membrane of the uterus 

 of the parent being regenerated during-, and after, each preg- 

 nancy. 



There are, however, two orders of existing monodelphous 

 Mcmimalia^ the nature of the placentation of which is not yet 

 fully made out. One of these is the Sirenia^ the placentation 

 of which is unknown. The other is the ill-defined and hetero- 

 geneous assemblage called Edentata. Some of the members 

 of this group certainly possess deciduate placentse, while, in 

 others, it appears questionable whether the decidua is, or is 

 not, developed. And, as this group, the Edentata^ is decidedly 

 the lowest of the whole division, I shall take it first in order, 

 while the Slrenia are arranged, provisionally, among the JVon- 

 deciduata. 



The Edentata, or Bktjta. — In these Mammals the teeth 

 are by no means always wanting, as the name of the group 

 would seem to imply ; but, when teeth are present, incisors 

 are either altogether absent, or, at any rate, the median in- 

 cisors are wanting in both jaws. The teeth are always devoid 

 of enamel, consisting merely of dentine and cement. As they 

 grow for an indefinite period, they never form roots ; and, so 

 far as our knowledge at present extends, those which first 

 appear are displaced by a second set only in some of the Arma- 

 dillos. The ungual phalanges of the digits support long and 

 strong claws. 



There are mammae upon the thorax, and sometimes, in ad- 

 dition, on the integument of the abdomen ; or, in the inguinal 

 region. 



The brain varies greatly, its hemispheres being sometimes 

 quite smooth, with a very small corpus callosum and large an- 

 terior commissure ; while, in other cases, the corpus callosum 

 is much larger, and convolutions appear upon the surface of 

 the brain. 



The Edentata are divided into the Pliytophaga^ or vege- 

 table-feeders, and the Entomophaga^ or insect-eating forms. 



