924 MB. E. LYDEKKEB, OK THE DENTAL EOEMULA OF [Nov. 14, 



and that the true homologue of pm. 4 is not developed in the 

 Marsupialia." 



In this passage, then, the great anatomist recognizes, firstly, that 

 the first true molar of Placentals belongs to the first series of teeth ; 

 and, secondly, that the fourth cheek-tooth of the Marsupials is a 

 persistent last (fourth) milk-molar. And it is merely in order 

 to obtain general recognition for these two important facts that 

 the present paper is chiefly written. 



The next amendment in the dental homology of the Marsupial 

 and Placental Carnivora was made by Professor A. Graudry ^, in 

 1878, who, struck by the resemblance between the teeth of the 

 Creodont and Marsupial Carnivora, applied the same formula to 

 both, thus making the lower dentition of Thylacinus i. 3, c. l,p. 4, 

 m. 3, or the same as that of Hycanodon and Canis. He then 

 pointed out that although the Creodonts difiered from Thylacinus 

 and its allies by a complete dental replacement, yet the former 

 likewise differed from modern land Carnivora by the circumstances 

 that all their three true lower molars were of a carnassial type, 

 and that they closely resembled the corresponding lower teeth of 

 the Thylacine. No attempt was, however, made to show why the 

 latter animal, in common with its kindred, should have four teeth 

 of this same cai'nassial type. 



In 1887 appeared a paper by Mr. 0. Thomas ", in which the 

 replacing tooth of the Marsupials was definitely regarded as 

 representing p. 4 of the Placental series, and was accordingly termed 

 the fourth premolar ; the second tooth of that series being regarded 

 as missing in the modern Marsupials. In this communication the 

 author suggested the use of the term " milk-premolars," in lieu of 

 milk-molars. Mr. Thomas's nomenclature of the Marsupial series 

 was adopted in the ' Study of Mammals.' 



It was some years after the appearance of the paper last referred 

 to that the researches of Messrs. Kiikenthal and Eose afforded 

 grounds for regarding all the teeth in advance of the replacing pre- 

 molar of modern Marsupials as milk-teeth, and the identification of 

 the true molar series as corresponding serially with the milk set 

 rather than with the premolars. To these discoveries I need not 

 refer further than to say that a useful summary of them is given 

 by Professor Osborn in the 'American ISTaturalist ' for 1893 \ 



I accordingly pass on to two papers by Seiior Florentino 

 Ameghino, in the course of which the remains of certain Marsupial- 

 like Mammals from the Tertiaries of Patagonia are described and 

 figured under the group-name of " Sparassodonta," In the first 

 of these communications ^ the animals in question are said to be 

 referable neither to the Carnivora Vera, the Creodontia, or the 



^ ' Les Encliainements, etc. — Mammiferes Tertiah'es,' pp. 13-19. 



- Phil. Trans. 1887, p. 447. Many of the views propounded here were 

 modified in a paper published in the Ann. Mag. Nat. Hist. ser. 6, vol. ix. 

 p. 308 (1892). 



' Vol. xxvii. pp. 493-508. 



* Bol. Ac. Cordoba, vol. xiii. pp. 259-452 (1894). 



