428 PLAGIAULAX. 



the generalization in question, we might a priori have ex- 

 pected to encounter at the near end among existing mar- 

 supials. 



The curious fact, that only lower jaws should have turned 

 up among the Stonesfield mainraalian remains has often 

 been the subject of speculation or remark. The same, to a 

 certain extent, has held good with the remains found in the 

 Purbeck beds. Among the determined fossils, lower jaws 

 predominate largely. But some upper rnaxillaries have been 

 met with, and, while writing, I have received intimation of 

 the discovery of more. Among the undetermined remains 

 there is a considerable number of other bones of small ani- 

 mals, many of them probably mammals, but they are seldom 

 or ever perfect. In these minute creatures, unless the bone 

 be complete, and, supposing it to be a long bone, with both 

 its articular surfaces perfect, it is almost hopeless, or at any 

 rate very discouraging, to attempt to make out the creature 

 which yielded it ; whereas the smallest fragment of a jaw 

 with a minute tooth in it, speaks volumes of evidence at the 

 first glance. This I believe to be one great reason why we 

 hear so much of jaw -remains, and so little of the other bones. 

 For, as an inferior maxillary is to the other bones of the 

 skeleton in the ratio of about 1 to 250, coeteris paribus, a large 

 number of these should be encountered for every lower jaw 

 that turns up. 'No indication has yet been met with at Pur- 

 beck of the bone of a good-sized terrestrial mammal. But I 

 do not consider the negative evidence in this case to be de- 

 cisive of their non-existence. The matrix of the so-called 

 ' Dirt-bed, No. 93,' by which most of the mammal remains 

 have been yielded, is a whitish-grey, fine-grained marl, full 

 of the exuviae of freshwater animals, hardening into a kind 

 of stone when the moisture is expelled by desiccation, but 

 very bibulous, and readily becoming pasty, a*fter immersion 

 in water. It has properly no claim to the designation of a 

 ' dirt-bed,' or ' ancient vegetable soil,' as there is rarely a 

 speck of vegetable matter to be seen in the numerous speci- 

 mens containing bone-remains which have passed through 

 my hands. 1 It appears to me to present more the character 

 of the deposit near the margin of a patch of fresh water, and 

 that the probable explanation of the association of so many 

 small bones of minute mammals and lizards is that they were 

 the floating objects most readily drifted to the margin by a 

 surface-ripple from wind, or by a wave-eddy. In India, in 



1 I am informed by the Assistant- i in 185J-, were of a dark colour and eon- 

 Secretary of the Geological Society, how- tained vegetable remains, together with 

 ever, that the hand-specimens of this freshwater shells. See Quart. Journ. 

 bed, in which Spalacotherium occurred | Geol. Soc. vol. x. p. 423. 



