412 J. C. Mansel-PIeydell — Geology of Dorset. 



able feature in the molluscous fauna of the Purbeck beds is their 

 similarity with those of the fresh-water Tertiary strata, and with 

 existing genera ; this resemblance is so great that, with the fossils 

 before us, and no evidence of their geological position, it would be 

 difficult to assign them to a definite geological epoch. 



At the commencement of the present century it was generally 

 supposed that the appearance of mammalia was deferred until the 

 Tertiary period, but this delusion was dispelled in the year 1814 by 

 the discovery of the jaw of a small marsupial in the Lower Oolites of 

 the Stonesfield Slate ; and, thirty years after, the tooth of another 

 diminutive mammifer Microlestes was discovered still further down 

 in the geological series, in the Upper Trias of Stuttgardt. The Upper 

 Trias of Somersetshire subsequently supplied jaws and teeth of two 

 or three marsupial species ; although no mammal had up to this 

 time been found between the other members of the Oolitic series and 

 the Tertiary, it was not unreasonable to suppose, that, as abundant 

 proofs existed of plants as well as insects, the balance of life was 

 carried on, as at the present day, by compensating influences, and 

 that Insectivora were present then as now to control the undue in- 

 crease of insect life. The presence of fossil Mammalia in the Middle 

 Purbeck was established by the discovery of an insectivorous mammal, 

 Spalacotherium tricuspidens, allied to the Thylacotherium of the Stones- 

 field Slate. In the year 1856, Mr. S. H. Beckles, F.R.S., stimu- 

 lated by these discoveries, proceeded to make a close and systematic 

 search in the fresh- water marl-beds, and with complete success. In 

 three weeks he brought to light, from an area forty feet long by ten 

 wide, and from a layer the average thickness of which was only five 

 inches, several species, many of which were new. They are the 

 subject of an exhaustive meraoir by Professor Owen, in the Palseonto- 

 graphical Society's works for the year 1870. No less than eleven 

 genera and twenty-seven species are described and figured ; of these 

 the genus Plagiaulax has engaged the greatest attention, and has 

 given rise to a controversy as to its being carnivorous or insect- 

 ivorous. The late Dr. Falconer thought it might be regarded as 

 possessing the type peculiar to the family to which Hypsiprymnus 

 belongs, but generically widely distinct from the existing Kangaroo- 

 rat : Flagiaulax Bechlesii equals the size of a squirrel. Professor 

 Owen on the other hand contends that the demonstration of the 

 carnivorous character of Flagiaulax is more ample than that of its 

 having been herbivorous. Small reptilian remains — Macellodus 

 Brodiei and Nothetes destructor — are found in the feather quarry, 

 (J.) chert beds of Mr. Austen's list. 



It is worthy of remark that of the twelve or fourteen species of 

 Purbeck mammalia not one is a member of the orders which are so 

 well represented in the Tertiaries, and all appear to be marsupials 

 allied to Australian groups. 



At Durleston Bay and at Eidgeway some of the beds are prolific 

 in insect-remains, especially those of the lower part of the series, 

 consisting, for the most part, of elytra or wing cases of Coleopterous 

 insects interspersed with some Neuropterous wings, and other relics 



