100 DE. B. A. BENSLET ON THE EVOLUTION 



dissents from this proposition, and believes the increased molar formula to represent a 

 primitive character inherited from Mesozoic forms *. 



In a former paper (1901 a) the writer expressed the opinion that the dental characters of 

 Mijrmecohius have been derived by retrogression from those of the normal Dasyuridse, 

 and this position has been amply confirmed by the examination of the extensive series 

 of specimens in the British Museum collection. Many of the characters of the incisors, 

 canines, and premolars, which appear at first sight to be primitive, are repeated in the 

 Peramelidfe, wliere they are undoubtedly the result of retrogression. The patterns of 

 the lower molars are directly derivable from those of the smaller Dasyurinae. In fact, 

 the only real difficulty in the derivation of the dentition relates to the patterns of the 

 upper molars, these being so extremely variable that the cusp-homologies are difficult 

 of determination. Nevertheless, even in them it is possible to recognize a ground-type 

 running through the variations, and this ground-type represents approximately the 

 })attern characteristic of the normal Dasyurinae. 



With reference to tl\e origin of the excessive molar formula in Myrmecoblus, it will be 

 seen that there are three possible explanations : — [a) That it is due to the retention of the 

 normally deciduous teeth ( Winge) ; {b) tliat it represents a primitive condition carried 

 over from the Mesozoic Mammalia (Owen, Leche, and others) ; {c) that it is due to 

 a simple reduplication of teeth from the posterior portion of the dental lamina, or to a 

 reappearance of formerly vestigial teeth in the same region. 



As to the possibility of a retention of the deciduous teeth, a description will be found, 

 on pp. 106-107, of a young specimen in the collection in which the posterior premolars 

 have not yet begun to develop ; and of three molariform teeth already formed, the first 

 are comparatively minute, while the second are approximately equal in size to the unworn 

 first molars of the adolescent animal. This appears to indicate that the minute first 

 molariform teeth represent decidvious premolars, which are afterwards replaced. It may 

 be observed that, even assuming a retention of the deciduous teeth, we would still have 

 to account for the occasional presence of an additional lower molar. 



With reference to Leche's suggestion that the increased formula is a primitive 

 character, the writer believes it to be counterindicated by the following facts : — [a) The 

 extreme variability of the dentition and the abundant evidence of retrogression from a 

 normal Dasyurine condition, added to the fact that there is an intimate correspondence 

 between Ilijrmecohius and the Dasyuridae in other respects, and more especially in the 



reduced incisor formula of o' indicate that the relations of the animal are in every Avay 



with the latter family. That this relation is, however, not an ancestral one, as the 

 increased molar formula would seem to indicate, is apparent from the fact that the 

 ancestral modifications of the Dasyuridse are to be found in Didelphyidae, which forms 

 present a more primitive condition of the incisor formula than is found in Myrmecoblus. 

 (b) The data collected by Bateson (1894) show that reduj)lication of teeth may occur 



* " Es scheint mir somit festzustehen dass das Myrmecobiiis-Gehiss, was die Form der Backen-Ziihne betiifft, 

 theilweise ro.lucirt ist, und demselben Typus wie dasjeuige der Dasyuridse und Ornithorhym-Jius angehiirt, dass 

 aber die grossere Aiizabl etwas Primitives, von Mesozoischen Siiugethieren Ererbtes ist." 



