GENERAL CHARACTERS 



177 



remarkable exceptions of Tatusia and Orycteropus, monophyodont ; 

 they are never rooted, but have persistent pulps ; except in some 

 fossil forms, they are always deficient in one of the constituents 

 which enter into the formation of the complete mammalian 

 tooth, the enamel ; and, at least among living forms, are never 

 present either in the upper or lower jaw in the fore part of 

 the mouth, the situation occupied by the incisors of other 

 mammals. 1 



The peculiar nature of the dentition in the aberrant Onjcteropus 

 will be noticed under the heading of that genus. As a rule, the 

 coracoid process of the scapula of the Edentates is more developed 

 than in other Eutheria. 



The degree of development of the brain varies considerably in 

 the different families, the 

 hemispheres being in some 

 cases almost or quite smooth 

 (Fig. 57), with a small corpus 

 callosum, and large anterior 

 commissure ; while in other 

 instances the hemispheres 

 are convoluted, and the 

 corpus callosum is larger. 



There is so great a differ- 

 ence in structure and habits 

 between some of the existing 

 animals assigned to this order 

 that, beyond the negative 

 characters just mentioned, 

 there seems little to connect 

 them. The Sloths and Anteaters, for instance, in mode of life, 

 general conformation of limbs, structure of digestive organs, etc., 

 appear at first sight almost as widely separated as any mammals. 

 Palaeontology has, however, thrown great light upon their relations, 

 and proved their real affinities. Perfectly intermediate forms have 

 been discovered in the great Ground Sloths of America, which have 

 the dentition and general form of the head of the Sloths, combined with 

 the limbs and trunk of the Anteaters. It is, indeed, highly probable 

 that the existing members of this order are very much differentiated 

 representatives of a large group, the greater number of which are 

 now extinct, and have become so without ever attaining a high 

 grade of organisation. The great diversity of structure in the 

 existing families, the high degree of specialisation to which many 

 have attained, the paucity of species and even of individuals, their 



1 In some few Armadillos the suture between the premaxilla and maxilla 

 passes behind the first upper tooth ; but in all other known members of the order 

 all the teeth are implanted in the maxilla. 



12 



FIG. 57. Upper surface of the brain of the Broad- 

 banded Armadillo (Xenurus ttnicinctus). The large 

 olfactory lobes are seen at the anterior extremity 

 (left of figure) ; the hemispheres have only three 

 sulci. (From Garrod, Proc. Zool. Soc. 1878, p. 230.) 



