OF THE AUSTEALIAN MAESUPIALIA. 101 



without reference to homology, while the observations of Thomas (1888) and Allen (1901) 

 show, further, that an excessive number of molars may be found as a variation in normal 

 Marsupials {Bettougia and Didelphys). (c) The minute size of the molars in Myrmecohins 

 and the elongation of the palate and lower jaw offer just those conditions favourable for 

 the intercalation of new teeth, or, as indicated above, their origin by redujilication from 

 the posterior part of the dental lamina. 



Unlike Leche, the writer sees nothing incompatible in the recognition of a retro- 

 gressive development of the already existing teeth and the synchronous addition of new 

 ones. The reduction of the function of an organ is not immediately followed to its 

 obliteration, the latter only taking place after a longer or shorter period of attempted 

 development. While, therefore, the molar teeth of Myrmecohim may be fouud in 

 a semi-reduced condition, the mere fact of their presence is sufficient evidence of their 

 tendency to develop in a normal way. It is accordingly reasonable to suppose that 

 under the favourable conditions of increased space in the molar region, the same 

 tendency as is seeu under less favourable conditions in Bettonyia and Didelphys to 

 produce new teeth should not only be present, but also be more strongly marked. In 

 fact, the existence of such a tendency is in evidence in the case of the antemolar teeth, 

 although the latter are admittedly less reduced than the others. Two of the British 

 Museum specimens already mentioned by Thomas (1888) show four instead of three lower 

 incisors, and Leche (1891) has described a specimen in which there is an additional 

 premolar in the left ramus of the lower jaw behind the posterior premolar. In the 

 Peramelidse w^e find several instances in which reduction of the caniaes is accompanied 

 by the appearance of new basal cusps, and an analogous case is seen in the Didelphyida^ 

 where in Caliiromys a retrogressive development of the external styles of the upper 

 molars [cf. PI. 5. fig. 27) is proclaimed by a reduction of the larger elements normally 

 present, and the development of a large number of smaller ones. 



No surprise need be expressed that such new teeth should resemble in their characters 

 the already existing ones. The results of Bateson (i 894) show that in the intercalation of 

 new teeth without definite homologies the same principle determines their patterns as 

 those of the normal teeth of the region in which they occur. 



With reference to the comparisons which have been made between the molars of 

 Myrmecobius and those of Ornithorhynchus and certain of the Multituberculata, there 

 is no doubt that certain resemblances exist, but they are of much too general a kind to 

 be interpreted as indicating affinities. A careful study of the patterns presented by 

 Plagiaulax, Microlesies, Ornithorhynchus, and Myrmecobius fails to reveal any evidence 

 of homologous cvisps. The fact, already referred to above, that the relations of 

 Myrmecobius are in most respects with the Dasyuridte, whose prototypal characters are 

 those of the Didelphyidae, strengthens the view that such resemblances as do exist are 

 merely the result of convergent dcveloi)inent. The molars of Myrmecobius dilfer from 

 those of the normal Dcisyuridae chiefly in lacking the angularity both of the general 

 contour and the constituent cusps, which, in the latter, is due to the mechanical precision 

 with which the teeth are fitted together. It is exactly in these obviously secondary 



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