1899.] THE MARSUPIAL AXD PLACENTAL CAENIVORA. 92o 



Dasyuridce, although stated to present resemblauces to eaeli oE these 

 groups. Apparently in all cases the palate is devoid of the uu- 

 ossified vacuities characteristic of existing Marsupials. In many 

 instances the upper incisors exceed the number occurring in modern 

 Placentals, one of the genera (Prothi/lacimis) having the same 

 incisive formula as in TJiylacinm, namely g. The cheek-teeth (as 

 shown in figs. 3, 5, and 6 of Plate LXII.) are also of a Marsupial 

 type, the total number being seven, of which the last four are 

 molariform. And in his first communication Seuor Ameghino 

 divides them according to the formula usually accepted for the 

 Marsupialia ; that is to say, into three premolars (p. 2, p. :-5, 

 and p. 4) and four molars. He also goes on to observe that while 

 the milk-dentition is more reduced than in the Carnivora, it is 

 less so than in the Dasyurida. The genus in which the reduction 

 is carried to the greatest extent is the one named Borhyana 

 (Plate LXII. fig. 3), in which only the canine and the fourth cheek- 

 tooth have vertical successors. On the other hand, in the other 

 genera (e. g. Prothylacinus, tig. 5, and Ampliiproviverra, fig. 6), 

 both the second and third cheek-teeth, in addition to the canines, 

 are thus replaced. In regard to the incisors there is no evidence. 



In the drawing (Plate LXII.) I have had the lower jaws of the 

 three genera mentioned figured alongside of those of the Creodont 

 genera Hycenodoa (fig. 1) and Pterodon (fig. 2) above, and of the Mar- 

 supial IJn/lacinus (fig. 4) below. And an inspection of these will 

 show that, whereas the jaws shown in figs. 1 and 2 have but three 

 molariform teeth, all the others have four. The general resem- 

 blance is, however, so striking between the whole series, that it 

 is almost impossible to conceive that the se\en cheek-teeth are not 

 serially homologous with one another in the sis genera. 



And this idea has been developed in Senor Ameghino's second 

 paper, published in the Society's 'Proceedings ' for the present year'. 

 Thus on page 556 he writes that he assigns to the teeth behind 

 the canines the progressive numbers 1 to 7^ since they are perfectly 

 homologous in the Placentals and Marsupials, the only difference 

 being that some teeth may belong to the first series in certain 

 genera (e. g. the fourth in Marsupials) and to the second in others 

 (e. g. the fourth in Placentals). 



This view is in fact the one advanced by Owen, when he said 

 that the fourth cheek-tooth of the Thylacine was a milk-molar 

 rendered permanent by the suppression of its vertical successor. 

 And looking at the number of forms described by Senor Ameghino 

 which serve in some degree to connect the Creodontia with the 

 Basyuridce, it appears to me, as already indicated, impossible to 

 avoid accepting the above interpretation. The fourth cheek-tooth 

 in the Prothylacinidce (Sparassodonta) indisputably belongs to the 



' Supra, pp. .5.55-571 ; I am not prepared to adtnit tbe Cretaceous age of 

 some of the specimens described therein. 



- This nomenclature had been long since proposed by Dr. H. Winge, Vidensfc. 

 Med. KjSbenhavn, 1882, p. 65. 



Proc. Zool. Soc— 1899, No. LX. 60 



