io6 NATURAL SCIENCE. april. 



We have yet another interesting discovery to chronicle before 

 concluding our notice of the mammalian palaeontology of South 

 America. This relates to the remains of numerous small mammals 

 presenting a more or less marked similarity to some found in the 

 Secondary and Lower Eocene rocks of Europe and North America, 

 and described as the Plagiaulacidae. We may observe that these 

 mammals (of which the Jurassic Plagiaiilax and the Eocene Neoplagi- 

 aulax are well-known examples) are characterised by having a single 

 large obliquely-grooved cutting tooth on either side of the lower jaw^ 

 which presents a considerable resemblance to the last lower premolar 

 of the Rat-kangaroo (Potovous) of Australia, and also by having an 

 equally large proclivous incisor on each side of the front of the same 

 jaw. It was long considered that these Plagiaulacidae were near allies 

 of the kangaroos, but later observations have discredited this view, 

 and have suggested that they were more nearly related to the 

 Monotremes. Now, the South American forms (which have been 

 imperfectly known to science for several years) also have a pair of 

 procumbent incisors and a very large cutting premolar-like tooth in 

 the lower jaw (Fig. 7) which may be vertically grooved ; and it has 



Fig. 7. — The right ramus of the mandible oi Abdcrites, 5 natural size.— After Ameghino. 



accordingly been presumed that they were closely allied to the Plagi- 

 aulacidae. The last-named tooth is, however, much larger in 

 proportion to the others than is the case in the Plagiaulacidae ; while 

 it is separated from the large incisor by five exceedingly minute, and 

 evidently functionless teeth, which are very different from the two or 

 three grooved premolars occupying the same position in the 

 Plagiaulacidae. Moreover, behind the large grooved tooth there are 

 three molars, in lieu of the two of the latter. Dr. Ameghino considers 

 that the large grooved tooth of these mammals, as represented by 

 Abdevites (Fig. 7), is the first molar, and not the fourth premolar; and 

 he accordingly also regards the large secant tooth of the Plagi- 

 aulacidae as a true molar. We are, however, by no means sure that 

 the resemblance between the Abderitidae and the Plagiaulacidae is 

 anything more than a superficial one ; and, if this proves to be the 

 case, the former family will indicate an entirely new group of 

 mammals, of which the affinities cannot, at present, be even 

 surmised. Whatever be the ultimate result, there can, however, be 

 no sort of doubt as to the great interest attaching to these peculiar 



