xxxvi Proceedings. {^March igth, igi2. 



been presented to the British Museum of Natural History. 

 One of the specimens was reported as having been found in 

 the China Sea, and was presented to the Manchester Museum 

 by the late Mr. R. D. Darbishire. 



Mr. C. E. Stromeyer, M.Inst. C.E., exhibited, and made a 

 few remarks upon, a piece of tramway rail, showing the ridges 

 and hollows due to the action of the wheels. 



A paper entitled, "The formal specification of the 

 elements of Stress in cartesian, and in cylindrical and 

 spherical polar coordinates," was read by Mr. R. F. 



GWYTHER, M.A. 



This paper is printed in full in the Memoirs. 



Ur, HiCKLiNG read a paper on the "Variation of 

 Planorbis multiformis." It was shown that the shell exhibited 

 every gradation from a perfectly flat type to one with a high 

 spire. The mean type is represented by a large number of 

 specimens, while the extreme types are scarce. The curve 

 representing the relative numbers of the various types is a typical 

 simple variation curve, thus proving that all the forms belong to 

 a single species. Great variation occurs in other characters of 

 the shells, and these variations appear to be independent of one 

 another. 



