102 DE. B. A. BEXSLEY ON THE EVOLUTIOX 



features that Myrmecohius resembles the plagiaulacid Multituberculata. As to what 

 kind of adaptations the teeth of the latter animals represent is wholly obscure. In the 

 writer's opinion the tabulate teeth of Ornithorhynchus are highly specialized, and 

 reioresent an adaptation for crushing the shells of small molluscs. They are comparable, 

 in a general wav, with those of the Trichechidce, or, better, the Sea-Otters. The excessive 

 crenulation of the molar margins in the specimen described by Stewart (1891, pi. 8), 

 Avhich enhances their multituberculate character, is undoubtedly secondary, and repre- 

 sents a development of much the same order as that seen in the teeth of the Suidae and 

 Ursidae. Without expressing an opinion as to the Multituberculata, the writer considers 

 the superficial resemblances existing between Myrmecohius and Ornithorhynchus to be 

 referable to different adaptations, and therefore not only secondary, but also much less 

 worthy of being designated as convergent developments than, for example, those seen in 

 the molars of the Plialangeridse and Primates, w^bere similar quadrituberculate teeth 

 have been evolved independently for similar purposes. 



Before considering the dental characters of Myrmecohius in detail, it may be observed 

 that the whole dentition is extremely variable, and that no adequate conclusions can be 

 formed from a study of single specimens. The variability is much more pronounced in 

 the upper molars than in any of the remaining teeth, there being in them not only no 

 coi-respondence between the homologous teeth of different individuals, but also none 

 l)etween those of opposite sides of the jaw in the same animal. The cause of the 

 variability is partly to be sought in the uneven mechanical wearing to wliich the teeth 

 are subjected, through the presence of particles of earth in the food. This is especially 

 noticeable in the case of the molars, where, as Leche (1^91, p. 151) has shown, the outer 

 parts of the lower teeth and the inner parts of the upper ones tend to be obliterated 

 Avith age, while the remaining parts are, in each case, only slightly affected. Beyond 

 this, however, there is a variability of an inherent kind which is not the result of wear, 

 but of the retrogression which is taking place throughout the dentition. It is interesting 

 to note that a similar tendency towards variability is seen under similar conditions in the 

 Peramelidse. 



Molar JPatterns. — The lower molars are much more constant in their characters than 

 the upper, and also depart to a less extent from the normal dasyurine tyjie, so that they 

 may be more conveniently described first. 



As already mentioned, apart from their minute size, the molars of Myrmecohius differ 

 from those of the Dasyuringe chiefly in lacking the angularity both of the general 

 contour and of the constituent cusps. The lower teeth (PL 6. fig. 9) are roughly 

 oval in section, and their cusps are either conical or slightly curved. The crown of an 

 unworn posterior tooth shows exactly the same number and arrangement of the cusps 

 as is seen in the teeth of normal dasyurine forms {cf. fig. 3), there being in all six 

 cusps, of which the three anterior together represent a trigonid, while the remaining 

 three represent a talonid. These two portions of the tooth are not differentiated as in 

 normal forms, the reason being partly that the crown-surface of the talonid is placed 



