146 Mr. Kenneth McKean. 



Mr. E. Saunders, of the Holmesdale Club, tells me he took 

 P. contecta some years ago at Flanchford, near Reigate, but it 

 does not appear to live there now. Bythinia tentaculata is 

 common in most ditches that contain water all the year round. 

 The operculum of this species fits the mouth of the shell with 

 such accuracy that if the animal be killed in boiling water and 

 then allowed to dry up within and the shell be held in the 

 warm hand, the expansion of the air causes the operculum to 

 fly off with a considerable report. B. Leachii, a very local 

 shell, may be obtained where one would hardly expect to find 

 an3'thing uncommon, viz., the little pond at the top of Coombe 

 Lane where a large portion of the Croydon dogs enjoy their 

 Sunday tub. Valvata piscinalis and V. cristata both occur in 

 the Wandle at Waddon ; the last named is decidedly uncommon. 

 It should be noticed that the shells of the Wandle are very 

 clean and somewhat smaller than those of our other streams. 



Hydrohia, of which we have one of the two British species, 

 viz., H. veiitrosa, occurs between Woolwich and Erith, in 

 ditches which are only filled at the top of the tide but which 

 never run quite dry. Assiminea Grayana inhabits the same 

 locality, but selects those ditches which leave it quite dry 

 about four hours each tide; it may then be found in such 

 countless m3Tiads as to completely blacken the surface of the 

 thick tenacious clay. When Hydrohia and Assiminea are 

 placed in a vase of brackish water supplied with Ulva, their 

 habits are seen to be widely different; the former will feed for 

 hours at a time and then roll off and lie motionless at the 

 bottom till again hungry. Assiminea immediately seeks the 

 air and wanders round the edge of its prison, even escaping if 

 not prevented ; it seems to wait for the rising of the tide to remind 

 it of feeding time, and will remain out of water for days till 

 obliged by hunger to descend in quest of food. 



Pulmonobranchiata, or air breathing water snails. — The 

 next family the Limnaeidae, is a large one, and is fairly 

 represented here. The beautiful Plaiwrbis nautileus is found 

 on Mitcham Common, Chipstead and Betchworth. At the 

 last-named place I took a monstrosity having the last whorl 

 separated. Plaiwrbis nitidus inhabits the sewage farm. P. 

 Cornells is ver}' local. Fine large shells of this species abound 

 on Wandsworth Common. The rare var.albiiia occurs at Kings- 

 wood. P. coiitplanatus, the most frequent of this genus, appears 

 more liable to distortion than any other shell ; instances of this 

 where the last whorl turns downwards and inwards, occur at 

 Pease Marsh, near Shalford. The ubiquitous L. peregra 

 flourishes alike in marsh, pond, ditch, and stream wherever 

 we choose to look. In clear rapid streamlets the shell is small 

 and the whorls of gradual increase and much compressed ; in 



