266 NELSON : ON THE LIMNJEIBM. 



of their shells. The Limncea glabra, L. sfagnalts, L. pahtstris, 

 and Planorbis corneus were eroded, whilst the Limncea peregra, 

 Planofbis marginatus and Physa hypnoriwi were not. Various 

 theories have been put forward at different times to account 

 for this erosion, the latest being that of Karl Semper, who 

 ascribes it as the work of a microscopic fungus. The latter 

 ditch and pond were near to Caville wood ; leaving here we 

 get to the end of Featherbed Lane. 



We again got into cross roads that seemed to lead any- 

 where but the way we wished to go, and the time was getting 

 along so that we did not do all the searching we might, but we 

 felt compelled to stop and search a pond in the corner of a 

 field which lies temptingly easy to get at ; here we found Physa 

 fontinalis and Planorbis marginatus. We were now getting near 

 to Wressle, which my companion was determined to visit, to 

 search for Pahidina coiiteda, and though we can see Wressle we 

 are compelled to describe a circle to get to it. After making 

 one or two fruitless efforts to get at the Fleetdike we were com- 

 pelled to go the usual way past the Castle along the Bubwith 

 road and across the fields. Getting to the dyke we lost no 

 time but began to search for Paliidina contecta, the most local 

 of our Yorkshire shells, this being at present perhaps the only 

 really Yorkshire habitat known. With careful searching we 

 soon found several examples each. I found a large Limncea 

 peregra and Mr. Collinge again found a single example of 

 Limcena glabra evidently away from home. Retracing our 

 steps across the fields we reached a pond near the dyke but 

 found the only shell to be Planorbis marginatus, and in another 

 pond in a pasture near to the Old Hall we found Limncea 

 peregra. We now got to the station where we had to sit some 

 time waiting for our train, and arrived at Leeds at about half- 

 past ten at night. 



We found that we had taken 13 species of Limmeidce. 

 Several common and widely distributed species, such as 

 Planorbis albus, P. contortus, and the two forms of Ancylus, all 



J.C, v., Jan., 1888 



