Prof. T. Rupert Jones— South African Shells. 409 



A Miocene elevation of the Weald and of East Mercian England, 

 such as has been here suggested, would allow that longer period of 

 time for the subaerial waste of the Chalk, which is required to 

 account for the angular flint fragments of the plateau-gravels, 

 especially as we see them on the hills above Aldershot, where the 

 materials have not been transported very far. On the Chalk Downs 

 above Ventnor we see them still in position, just as they are left by 

 the solution of a chalk matrix with very little argillaceous material. 

 It seems probable that the materials thus furnished may have 

 acquired the subangular form which they exhibit in the plateau- 

 gravels of the Southern Drift during their transport northwards in 

 Pliocene times, owing perhaps to accentuation of the anticline of 

 the Weald at that period, and its elevation into a more definite hill- 

 range, causing an increase of precipitation and a greater volume and 

 rapidity of flow, with a correspondingly greater transporting power, 

 of the rivers which flowed from it, to convey the flinty materials of 

 the plateau-gravels, along with the debris of Neocomian rocks. 



These are little more than suggestions, but perhaps worthy of 

 consideration. I do not think they have been sufficiently considered 

 in connection with the general problem ; and this may be a justifica- 

 tion for the appearance of this short paper. 



YIII. — On Some Small Bivalve Shells from the Karoo Formation, 



South Africa. 



By Professor T. Rupert Jones, F.R.S., F.G.S., etc. 



AMONGST the specimens sent many years ago, from South 

 Africa, by the late Mr. Andrew Geddes Bain, to the Geological 

 Society of London, are two small blocks of greenish-grey hard 

 mudstone or shale, very slightly calcareous ; one bed-plane in each 

 specimen bears upon its surface numerous valves of one or more 

 forms of small Lamellibranchs, closely resembling the shells of the 

 genera Cyvena and Cyclas in contour. 



The longest axis of one of the largest and best-preserved individuals 

 is 7 millimetres, the shorter 5 mm. A smaller, well-shaped form 

 measures 5 mm. by 4 5 mm. Only in a few instances has it been, 

 found possible to develope these little valves from their matrix, so 

 as to show their contours clearly and accurately. 



Within certain limits, these little shells vary much in size and 

 contour, as the following measurements numerically express in mm. : 



From the anterior to the From the umbo to the ventral 



posterior margin of valve. margin of valve. 



3 x 2± 



U x 2i 



4| x 3^ 



5~ x 4J 



5x5 



6x4 



6x5 



7x4 



7x5 



7 x b\ 



8x5 (lengthened by crush). 



