XIII 



PHYLUM CHORDATA 



547 



molars. A good example of the diprotodont arrangement is the 

 Kangaroo (Macropus, Fig. 1200), which has the dental formula 



3 l 



%. -, c. -, r . 2 , ... 4 



2 4 



-, m. . = 34. 



The canine is very small and early lost. Of the polyprotodont 

 forms the Australian Dasyure or Native Cat (Fig. 1144) has the 

 formula 



. 4 1 

 i. , c. p. 



2 4 

 , m. = 42 ; 



and the American Opossum (Dtidphys) (Fig. 1202)- 



.513 4 

 -, 



7t e.^p.=,m.- A =50. 



The Edentata, as noticed in the outline of the classification, 

 though not by any means all toothless, always have some defect 

 in the dentition ; when teeth are present in the adult the anterior 

 series are absent and the teeth are imperfect, wanting roots and 

 devoid of enamel. The tooth-characters differ widely in the 



FIG. 1203. Section of lower jaw and teeth of Orycteropus. (After Owen. 



different groups. In the Sloths there are five teeth above and 

 four below on each side ; no second series is known. In the 

 American Ant-eaters there are no teeth in the adult. In the 

 Armadillos, on the other hand, the teeth are numerous, though 

 simple and rootless, and, in one genus at least, two series occur. 

 In the Scaly Ant-eaters there are no teeth. In the Cape Ant-eaters 

 (Fig. 1203), again, there are numerous teeth which are heterodont 

 and diphyodont, and have a peculiar structure, being perforated 

 by numerous minute, parallel, vertical canals ; the pulp of each 

 tooth, entire at its base, is divided distally into a number of parallel 

 columns. 



In the Ungulata the dentition is heterodont and diphyodont. and 

 the teeth are very rarely devoid of roots. In the Artiodactyla the 

 pre-molars and molars differ from one another in pattern ; the 

 first upper pre-molar is almost always without a milk predecessor. 



