K('7V : Snails and Spiders on Towt-rs. 343 



remarkable situation ; and one is naturally led to inquire as to 

 the orig-in of the colony. In this connection we may recall that 

 early in 1890 Mr. Heathcote saw a fine specimen of Limncea 

 triincatula — a water-snail — crawling- on the top of the tower of 

 St. James' Church, Preston, 90 or more feet from the ground. 

 As reg-ards land-shells, Captain Farrer has noted the occurrence 

 of the beautiful little Helicoid snail Vallonia pulchella on the 

 moss-g-rown roof of an old mill at Bassenthwaite ; Mr. S. C. 

 Cockerell has recorded the finding of Balea perversa and Cary- 

 chium minimnni high up under the clerestory windows of the 

 Cathedral at Beauvais ; and further the late Mr. E. L. Layard 

 discovered among the collections of Wollaston a box containing 

 a lot of fine adult Helix muralis, evidently collected by 

 Wollaston, and labelled in his well-known handwriting : ' from 

 the top of St. Peter's at Rome.' In some cases of this kind it 

 is conceivable that molluscs spreading generation after genera- 

 tion by ordinary progression may ultimately reach the odd 

 situations in which they occur ; this, however, is obviously 

 impossible in the case of the Litnncea ; and though it is difficult 

 to exclude the possibility of capricious action by man, it is 

 probable, I think, that we see in most of the cases the result of 

 accidental dispersal by winged creatures, presumably birds. 

 Daws build in the tower at Preston, and it is suggested that 

 one of them brought up the water-snail on a stick used for its 

 nest ; similarly it occurred both to Mr. Cockerell and Mr. 

 Layard that the colonies to which they refer had probably been 

 established by pigeons, which are supposed to have carried up 

 the molluscs on their nesting materials. Captain Farrer, more- 

 over, believed his Vallonia to be bird-carried, since he failed to 

 find the creature on the ground surrounding the mill. We have 

 a further reference to St. Peter's by Dr. McCook, who records 

 the finding of a number of spiders of the genus Epeira 

 \Araneiis\ in their orb-webs on the topmost railing of the dome. 

 Here, however, we have not to rely on accidental transport, 

 since the aeronautic habits of spiders, more especially the 

 young, furnish them with an efficient means of dispersal. 

 Taking up an exposed position facing the breeze, elevating the 

 hind-body, and allowing silk to be carried from the spinnerets, 

 they are soon afloat, and may be conveyed to great distances as 

 well as to great heights. This is a subject which much 

 interested our celebrated Lister, who tells us that he saw some 

 sailing spiders at York in 1669 with their lines entangled on the 

 pinnacles of the highest steeple of the Minster. 



1903 September i. 



