1893.] MB. M. r. WOODWABD OS MAMMALIAN DBNTITIOJiT. 451 



specimens in which the teeth were ah-eady cutting the gvim and 

 far too old, taken alone, to determine the true tooth relationships. 

 His principal conclusion is that the 3rd incisor, on account of its 

 lateness in cutting the gum, must belong to the 2nd dentition; 

 this, as I shall show later, is quite erroneous. 



II. — Obsertations ok the Specimens. 



Having a very fine series of Macropid embryos at my disposal, 

 some from Prof. Parker's collection and others received through 

 the kindness of Mr. Fletcher of Sydney and Dr. Symington of 

 Edinburgh, I propose to commence my tooth studies with that 

 group, as being one of which I can give the most complete 

 account. 



The following forms have been investigated : — 



Mcxropus giganteiis, 9 specimens. 



„ hennettii, 1 specimen. 



„ eugenii, 2 specimens. 



„ bracJiyurus, 2 specimens. 

 Petrogale j^enicillata ? , 6 specimens. 



„ sp. inc., 2 specimens. 

 Betiongia lesueuri, 1 specimen. 

 •^^yprymnus rufescens, 2 specimens. 



Unfortunately, in the case of the Petrogales I am somewhat 

 uncertain about the species, as the embryos collected by Mr. 

 Fletcher of Sydney seem according to his account to be P. 

 penicillata, while the same name was given me by Dr. Symington 

 for his specimen ; nevertheless, I have no hesitation in saying that 

 they are two distinct species, both, however, being Eock- Wallabies, 

 As a rule, the exact species would not perhaps matter very much, 

 but in this case it is a very vital matter, as the P. penicillata of 

 Mr. Fletcher was the most interesting and least specialized of all 

 Macropids with regard to its tooth arrangement, so much so that 

 I shall commence my observations on that form. 



Before proceeding to the detail observations, I should wish to 

 enforce the importance of Prof. Huxley's method of observation 

 by dissection under clove-oil, given in my previous paper (P. Z. S. 

 1892, p. 40). This method, employed side by side with examination 

 by means of serial sections, I hold to be very important and assists 

 one materially in interpreting the latter, although taken alone it may 

 be very misleading. One may simply clarify and sketch the jaw 

 before decalcifying and sectioning, and in this way gain a good idea 

 as to what teeth, especially vestigial ones, are present. Or, better 

 still, where one can afford to utilize both halves of the head of the 

 foetus, one can carefully dissect one half in oil of cloves and so 

 obtain a complete model of the developing teeth (generally con- 

 sisting at this stage of enamel organs only), both in relation to 

 the gum and to the maxilla and prem axilla, while one obtains a 

 complete series of sections of the other half. 



