Ojven — New Purheck Mammal. 199 



But, indeed, as respects the controversy as to the comparative 

 influence exercised by marine or atmospheric erosion in moulding 

 our present land-surfaces, an equally vast lapse of time -must in 

 either case be assumed. The object of this paper is simply to suggest 

 that the two denuding agencies have been always at work upon the 

 surface of the earth, and that there is ample reason to consider the 

 one to have produced effects quite as considerable as the other. 



II. — Desgription of Part op the Lower Jaw and Teeth of a 

 SMALL Oolitic Mammal {Sttlodon^ pusillus, Ow.) 



By Professor Owen, F.E.S., F.G.S., etc. 

 (PLATE X., Figs. 1,.2.) 



I HAVE been favoured by the Eev. Peter B. Brodie, M.A., F.G.S., 

 with part of the lower jaw, including eight back teeth (PL X., 

 Fig. 1, natural size), of a small mammal, nearly allied to Spalaco- 

 therium tricuspidens, Ow., and from the same formation and locality, 

 viz., the Marly bed Upper Oolite, Purbeck, Dorsetshire. 



The part of the lower jaw is embedded in a small block of the 

 matrix, with the outer surface exposed : it includes the proportion 

 of the ascending ramus supporting the coronoid process, a film of 

 which only remains in the depression of the matrix, mainly indicat- 

 ing its size and shape, and so much of the horizontal ramus as 

 includes the alveoli of the nine posterior teeth, eight of which are in 

 situ. The articular and angular processes, and the fore part of the 

 ramus, have been broken away, and there is no indication in the 

 matrix of the entire ramus having been imbedded therein ; so it may be 

 inferred, therefore, that the mutilation took place prior to imbedding. 

 Enough, however, has been preserved to demonstrate the class- 

 characters of the animal to which the fossil belonged, and to enable 

 us to add another genus and species to the small category of mam- 

 malia of the Mesozoic period. 



The continuous unity of bone at the part of the mandible which 

 would show most of the sutures in a lacertian ja^ — the height, breadth, 

 and contour of the "processus coronoideus " — and the implantation 

 of one, at least, of the teeth by two fangs in a double socket, concur 

 in testifying to the warm-blooded, air-breathing, viviparous, and 

 lactiferous class of the animal. The base of the coronoid process 

 shows the raised boundary of the lower part of the depression for the 

 insertion of a temporal muscle of mammalian proportions. The 

 lower margin of the ascending ramus has a degree of thickness and 



this question, which views he had previously disputed. " From the description," he 

 says, " given by Mr. Scrope, Sir C. Lyell, Sir R. Mui-fchison, and other competent 

 authorities, it plainly appears that the valleys of Auvergne were excavated not at one, 

 but at several successive periods— or, more correctly speaking, that although water 

 was instrumental in their formation, yet that they must have been scooped out, not 

 by any violent movement or sudden passage of a flood over the country, but bi/ the 

 long-continued action of the rivers now in existente." 



1 STvKos, pillar ; oSovs, tooth. 



