926 MR. E. LTDEKKER 0]vr THE DENTAL rORMULA. or [jSToV. 14, 



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

 there any evidence that this tooth ever had a successor in allied 

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

 question could possibly be answered in the affirmative is that of 

 the Purbeck genus Triconodon (Triacanthodon), 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. Prom this we pass to Prothylacinus and Amphipt'oviverra 

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

 observations are trustworthy) only the canine and the second and 

 third cheek-teeth are replaced; to Borhyo'na, in which replacement 

 is restricted to the canine and third cheek-tooth; then to -DicZeZ^^^s, 

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

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

 the same tooth is replaced in utero. 



Accepting, then, the foregoing interpretation, namely that the 

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

 Prothylacinus, and Thylacinus 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 I 

 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 Hycenodon will then stand as 

 follows, viz. : — 



i. 1 .i.2 . i. 3 c. 1 pp. 1 .pp. 2 .pp. 3. pp. 4 m. 1 . m. 2 .m.3 

 i. i. .i.'Z . t. 3 c. 1 pp. i .pp. 2 . pp. 3 .pp. 4 on. I . m. 2 . m. 3 * 



That of Borhycena will be : — 



? c.\ mp. 1 .pp. 2 . pp. 3 . mp. 4 )fi. 1 . m. 2 . m. 3 



i. 1 . t. 2 . i. 3 c. 1 mp. 1 .pp. 2 . pp. 3 . mp. 4 m. I .m.2 . m. 3 

 ^ Phil. Trans. 1887, pi. xxvii. fig, 10. 



