. A manual of zoology. Zoology. ir. VERTEBRATA: MAMMALIA, EDENTATA. 635 dentition, on the other hand, has undergone a jirogressive, diver- gent development, so that the distinctions are much more pro- nounced than in the marsupials, and hence of importance in differentiating the orders. Order I. Edentata. A few families, poor in species, are united under the name Edentata because teeth are absent or, as is more usually the case, are markedly degenerate. Persistent functional incisors are lack-. FiG. 659.—Cotyledonary placenta and embryo of cow. (From Balfour, after Colin.) C, cotyledons of ute
. A manual of zoology. Zoology. ir. VERTEBRATA: MAMMALIA, EDENTATA. 635 dentition, on the other hand, has undergone a jirogressive, diver- gent development, so that the distinctions are much more pro- nounced than in the marsupials, and hence of importance in differentiating the orders. Order I. Edentata. A few families, poor in species, are united under the name Edentata because teeth are absent or, as is more usually the case, are markedly degenerate. Persistent functional incisors are lack-. FiG. 659.—Cotyledonary placenta and embryo of cow. (From Balfour, after Colin.) C, cotyledons of uterine, C'^, of feetal placenta; C'/i, chorion; U, uterus; T, vagina. ing, canines but rarely occur {Bradyjnis); molars may be present, sometimes in great numbers (Priodon gigas, the large armadillo, has about a hundred molars), but they are poorly rooted, prismatic, without enamel, and usually monophyodont. Since the aardvark {(Jryctero^ms) and Tatusia have a heterodont milk dentition in embryonic life in which incisors occur, and fossil edentates {Entelops) with complete dentition are known, the absence of a replacement of the teeth is to be exjilained by degeneration, which may affect other jiarts, and is to a certain the reason for the low 2>osition accorded these forms. The great number of sacral vertebriB is striking, being as many as thirteen in some armadillos. The placenta is very variable, being diffuse, discoidal, or zonary in different species. The group is essentially tropical, but one species. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Hertwig, Richard, 1850-1937; Kingsley, J. S. (John Sterling), 1854-1929. New York, H. Holt and Company
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