fl26 MK. R. LTDEKKEB Oy THE DENTAL FORMTTLA OF [Xov. 14, 



first series, as it does in Thylacinus ; and we have now to ask, is 

 there any evidence that this tooth e\ er had a successor in allied 

 forms ? The only instance with which I am acquainted wliere this 

 question could possibly be answered in the affirmative is that of 

 the Purbeck genus Triconodon {Triacantliodon), in which, as shown 

 by Mr. Thomas \ there are at least seven cheek-teeth, of which the 

 fourth has a vertical successor. And it appears to me highly 

 probable that we have in this genus an ancestral type of Marsupial 

 in which all the first four cheek-teeth were replaced, as in the 

 Creodonts. From this w-e pass to Prothylacinus and Amjyhiproviverra 

 of the Patagonian Tertiaries, in which (if Senor Ameghino's 

 observations are trustworthy) only the canine and the second and 

 third cheek-teeth are replaced; to Borliycfna, in which replacement 

 is restricted to the canine and third cheek-tooth; then to l)ifWj:)%s, 

 in which only the third cheek-tooth has a successor and that at a 

 fairly advanced stage of life ; and finally to Tliylacinus, in which 

 the same tooth is replaced in iitero. 



Accepting, then, the foregoing interpretation, namely that the 

 seven lower cheek-teeth respectively met with in Canis, Hyivnodon, 

 Prothylacinus, and Thykicinus are serially homologous one with 

 the other, I come to the main object of my paper, that is to say, 

 to the formula we must adopt in order to indicate this. When 1 

 first considered the subject, I thought it would be necessary to 

 adopt the plan proposed by Seiior Ameghino, and to term the 

 teeth respectively 1 to 7. If this view were adopted, it would, 

 however, be necessary to use the term " cheek-teeth " in place 

 of " molars," as the latter has a special restricted signification. 

 Were we starting de novo, I think this would be the better course ; 

 but it is exceedingly inconvenient to interfere with the accepted 

 use of familiar terms, and Mr. Thomas has suggested to me a way 

 out of the difficulty which involves very little change. 



If we agree to call the first four cheek-teeth of all the animals 

 under consideration " premolars," as coming in advance of the 

 " molars," which never have successors, then we may designate 

 those that belong to the first series as " milk-premolars," aud 

 those of the second series as " permanent premolars," with the 

 respective symbols of mp. and pp. 



The adult dental formula of Hyoniodoa will then stand as 

 follows, viz, : — 



i. 1 . z. 2 . /. 3 c.\ pp. 1 . pp. 2 . pp. 3 . pp. 4 m.\ . m. 2 .m.S 

 I. 1 . t. 2 . «. 3 c. 1 pp. 1 .pp. 2 . pp. 3 .pp. 4 M. 1 . m. 2 . m.3 ' 



That of Borhycpna will be : — 



? c. 1 mp. 1 .pp. 2 . pp. 3 . mp. 4 »/.. 1 . m. 2 . m. 3 



i. I .i.2 ,i. 3 c.l mp. 1 .pp.2 .pp. 3 . mp. 4 m. I . m, 2 . >n. 3' 



' Phil. Trans. 1887, pi. xsvii. fig. 10. 



