336 Proceedings of the Uoyat Irish Academy. 



in the Museum of the Eoyal Dublin Society, the arrangement differs 

 slightly from this, for there the suture on the right side is present, 

 though much smaller than that figured above, whilst on the left side 

 a very small bar of bone, not more than one-twentieth of an inch in 

 width, runs across from the frontal to the maxilla, and prevents the 

 contact of premaxilla and frontal. 



As there are only the two skeletons of the Koala in Dublin, I cannot 

 say whether this suture is characteristic of the genus; but in the drawing 

 of the skull published by Owen'^ there is no appearance of it, nor in 

 the various allusions to it which I have met with can I find any notice 

 of it. On the contrary, it is generally laid down as a character of the 

 marsupials, that the premaxilla never quite reaches the frontal, so 

 that I am inclined to think that its presence in these two specimens 

 is due rather to individual variation. 



As regards the degree of extension of the premaxilla on the upper 

 surface of the skull, there is a good deal of difference amongst 

 Didelphs, as may be seen by the figures which I have drawn from 

 specimens in the Museum of Anatomy and Zoology. Thus Phas- 

 colomys fossor (Plate XIII., fig. 8) has the largest premaxilla,^ which 

 exceeds that in P. latifrons, Owen (Plate XI., fig. 4). Xext in order 

 would seem to come Phalangista vidpina, Desm. (Plate XIII. , fig. 9), 

 where the premaxilla is of a massive character, like that of Phascolo- 

 mys ; then would come Sahnaturus ualahatus, Less. (Plate XII., fig. 6), 

 and its ally Macrojms gigantea, Shaw (Plate XII., fig. 5), which 

 scarcely, if at all, proportionally, exceeds Sarcophilus ursinus, Geoff. 

 (Plate XIII., fig. 10) ; and finally, there is Didelpkys ccmcrivora, 

 Gomel (Plate XII., fig. 7), in which there is about the smallest extent 

 of premaxilla, at least in the series which I have examined. There is, 

 however, in the Museum a skeleton of Petaurus ariel, Gould, in which 

 the premaxilla seems to reach very nearly, if not quite, up to the 

 frontal; but the ossification of the skull has been so complete that the 

 sutures are almost entirely obliterated, and I have not been able, in 

 spite of careful examination, entirely to satisfy myself of their contact. 



In comparing the skull of the Koala with those of other marsupial 

 genera, we may notice that, while possessing strongly marked 

 characters of its own, it presents many features which are repeated 

 in the other forms. Thus we notice the parallel zygomata, as in 

 Nototherium ; the large tympanic buUte, like those of Petaurus ; the 

 large par-occipital processes, which resemble those in the Kangaroo, 

 to which there is a further likeness in the rounded premaxillse ; the 

 vertically placed supra-occipital, as in Phascolomys, and the truncated 

 nasals resembling those in Sarcophilus. 



A premaxillo-frontal suture does not appear to be common 

 amongst mammals, nor, indeed, amongst vertebrates. It is to be 



- "On the Fossil Mammals of Australia," Fhil. Trans., vol. clxii. part 1. 

 * Owen says that Phascolomys has the largest premaxilla, but he seems not to 

 have met with a Koala with a premaxillo-frontal suture. 



