LAND AND FRESH WATER MOLLUSCA OF THE EAST RIDING. 1 27 



visits of the Goole Scientific Society to Brough and Welton 

 Dale in 1876, and the work of the Rev. W. C. Hey in the 

 Filey district, and of Mr. J. D. Butterell near Hull about the 

 same date. More recently various localities in the south 

 have been frequently visited b)' Hull naturalists. The coast 

 from Filey to Bridlington, the neighbourhood of Beverley, 

 and the southern extremity of the Wolds have all been 

 thoroughly searched, but, with the exception of a few places 

 along the Driffield and Malton Railway, the central and 

 northern parts have not received any attention. It is highly 

 probable that careful search in these parts, especially along 

 the northern and western slopes would add several species to 

 the list, e.g., Helix lapicida, H. fusca, Hyalinia glabra, 

 Hy. excavata, Azeca tridens, Acicula lineata, Cyclostoma 

 elegans, H. lamellata, all of which are found in the North 

 Riding. From the present records, many of the more 

 primitive species appear to have been entirely driven out of 

 the East Riding by more highly organised forms. Pupa 

 anglica, however, still exist in small numbers on the cliffs 

 of Filey and Speeton. 



The Plain of Holderness resembles Derwentland in the 

 number of its watercourses and the abundance of aquatic 

 species, though the ditches of the boulder clay on the east 

 are not very productive. In the hollows of the boulder clay, 

 however, where the agricultural drains traverse the sites of 

 former marshes (e.g., between Marton and Aldborough), the 

 conditions are similar to those which prevail in the valley of 

 the Hull, and consequently aquatic forms are well distributed 

 throughout the district. As large areas lie below the level 

 of spring tides, and nearly the whole of the surface drainage 

 finds its way to the sea via the River Hull, the importance 

 of maintaining an unobstructed passage for it has been 

 appreciated from the earliest times. Mr. J. D. Butterell's 

 observation that the constant cleansing of ditches and drains 

 prevented the arrival at maturity of the larger species is 

 confirmed by Mr. F. W. Fierke (Fretwell's Guide, p. 56), 

 and probably the same fact may account for the absence of 

 some ; cleaning an agricultural drain involves not only the 

 cutting and removal of weeds, but the periodic excavation of 

 the entire bed of the stream. 



Forty-two species of fresh water mollusca have been 

 recorded for this division, four more than for Derwentland, 

 though the advantage is somewhat doubtful as the occurrence 

 of Viviparus viviparus, Unio tiimidus, and Unio pictorum 

 needs confirmation. Pisidium milium, Segmentina nitida, 



