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Show 926 MR. R. LYDEKKER ON THE DENTAL FORMULA OF [Nov. 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 (Triacanihodon), 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 m e 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 Amphiproviverra 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 Borhycena, in which replacement is restricted to the canine and third cheek-tooth; then to Diclelphys, 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 m y paper, that is to say, to the formula we must adopt in order to indicate this. W h e n 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 m e 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 " inilk-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. 1 . i. 2.t. 3 c. 1 pp. 1 .pp. 2. pp. 3. pp. 4 m. 1. m. 2 . m. 3 ' That of Borhyama will be :- ? _cj. mp. 1 .pp. 2Llpp. 3 . mp. 4 m. 1 . m, 2 . m. 3 i.l.i.2.i.3 c\ mp. 1 .pp. 2 .pp.3 . mp. 4 m. 1 . m. 2 . m. 3* 1 Phil. Trans. 1887, pl. xxvii. fig. 10. |