SUCCESSION OF THE TEETH IN THE DASYURID2E. 
449 
beds of Swanage, Dorsetshire.* An examination of this solitary lower jaw shows 
■what is, considering the immense antiquity of the species, rather a startling fact, 
namely, that it had an absolutely identical tooth-change to that found in modern 
Marsupials. This fact is shown most clearly by the typical specimen, which happens 
fortunately to be in precisely the right condition to show it, namely, with the milk- 
premolar still in its place in the jaw, while the permanent pm 4 ' is clearly visible buried 
in the bone beneath (see Plate 27, fig. 10). The milk-premolar is not small and 
nearly functionless, as it is in Phascologale, but is nearly as large as, and is very 
similar in shape to, the first molar standing just behind it. This ancient and remark¬ 
able fossil gives us, therefore, the one stage earlier than the abnormal Phascologale 
above described, having, on the presumption that its upper jaw resembled its lower, 
12 3 4 
the full premolar formula of P.M. ‘ ' all the teeth equally well developed, and 
the fourth one with a large and functional milk predecessor.t 
We may now consider this history of the evolution of the premolars of the 
Dasyuridm as fairly proved, and may represent it diagrammatically as follows :—■ 
Fig- 1- 
p. M- 
Process. 
Examples. 
( 
1 
2 
3 
T 
4 
I. 
V 
V 
V 
V . 
w 
Complete set. 
Triacanthodon. 
II. 
V 
V 
V 
V _ 
HP 
. Reduction in size of pm 2 . 
. Abnormal Phascologale. 
III. 
V 
o 
V 
V . 
HP 
Loss of pm 3 . 
Ordinary Phascologale. 
IV. 
V 
o 
V 
V * 
. Reduction in size of pm 4 . 
Phascologale apicalis, &c. 
V. 
V 
0 
V 
o • 
. Loss of pm 4 . 
. Dasyurus and Sarcophilus. 
In this diagram the permanent teeth are represented under their respective serial 
numbers by a V if present, and by an O if absent, while the milk-tooth is similarly 
shown by a shaded U if present, but is altogether unrepresented if absent. 
We will now pass from these details of individual species to the larger question as 
to the steps by which the primitive ancestral set of Mammalian teeth has become 
* This specimen is also referred to by Professor Flower in an address to tbe Odontological Society 
(‘ Odontol. Soc. Trans.,’ vol. 3, 1871, p. 220), and he there, on the then unverified assumption that it 
possessed a changing pm 4 , made certain remarks on the probable direction of the evolution of a 
Diphyodont dentition—remarks which all the evidence at my disposal most fully bears out. 
f As Professor Flower has pointed out in his paper just quoted, the type-specimen of Triconodon occisor, 
Owen (figured op. cit., PI. IV., fig. 2), found in the same beds, also shows traces of having had a changing 
pm 4 , the latter tooth being very markedly retarded in development, as if a milk predecessor had only 
just been lost from above it. There seems, in fact, to be every reason to suppose that all the Purbeck 
Polyprotodonts had a similar tooth-change, judging from such indications as may be gathered from the 
relative positions of the teeth. 
MDCCCLXXXVII.— B, 
3 M 
