The name that has been applied to this group is inappropriate
inasmuch as many Edentates have teeth. It is, however, by a number of small tooth-characters that the order can be defined. Thus if teeth are present they are simple in structure, without enamel in the adult condition, though a rudimentary enamel-organ has been discovered in an Armadillo. The teeth, moreover, are not found in the anterior part of the mouth, and they grow from persistent pulps; neither is there much differentiation among them. It is not possible, however, to speak of the Edentates as quite homodont, since in Orycteropus there are large cheek-teeth; but there is at any rate not a marked heterodonty in that or in any other Edentate. It used to be said that the Edentates were monophyodont. But the Armadillo Tatusia was subsequently found to possess a second suppressed dentition, and after this discovery Mr. Thomas proved that Orycteropus is also diphyodont. Since then other Armadillos have been shown to be diphyodont; and the whole group therefore, so far as concerns those members that have teeth, may in all probability be regarded as typically mammalian in this respect.
These characters are slender enough, but there seem to be no others by means of which the members of this order can be satisfactorily linked together. The fact is, that we have here a polymorphic order which contains in all probability representatives of at least two separate orders. We have at present a very few, and these perhaps highly modified, descendants of a large and diverse group of mammals. For convenience' sake they will be all treated of under the head of Edentata.
Although for the probable reasons already stated it is a hard matter to frame such a definition as will include all existing Edentates, it is easy enough to define two groups in this heterogeneous order; to define one group we should say, rather, and then to regard the leavings as forming another not so easily definable a group.
The perfectly-definable group is that which includes the American Anteaters, the Armadillos, and the Sloths. In all these creatures, which may certainly be regarded as representing on their own account as many family types, there are a number of important and highly-characteristic anatomical features which they share in common. So exceedingly different are these three types in general appearance and (correlated with that) in way of life that these common characters acquire increased importance.
Fig. 89.—Great Anteater (Myrmecophaga jubata). A, Side view of twelfth and thirteenth thoracic vertebrae. B, Posterior surface of second lumbar vertebra. C, Anterior surface of third lumbar vertebra, × ⅔. az, Anterior zygapophysis; az1, az2, az3, additional anterior, articular facets; cc, facet for capitulum of rib; m, metapophysis; pz, posterior zygapophysis; pz1, pz2, pz3, additional posterior articular facets; t, transverse process; tc, facet for articulation of tubercle of rib. (From Flower's Osteology.)
The first of these characters is the series of additional zygapophyses on the posterior dorsal and lumbar vertebrae; these are very clear in the Anteaters and Armadillos; less clear, but still obviously represented, in the Sloths. In the second place, they all possess a clavicle, rudimentary, it is true, in the Great Ant-bear, but still present. Thirdly, the testes are abdominal throughout life, a character which they share with such lowly-organised animals as the Monotremata and the Whales. Finally, and this is by no means a matter to be overlooked, not only are all the existing members of this group American in range, but there is no evidence to prove that they have ever existed elsewhere. No European or Old-World
representatives have as yet been discovered which can be referred to the Anteater, Armadillo, or Sloth type with certainty.[[94]]