SMITH : ON BRITISH HYDROBI^E. 145 



tentacles may, however, be just a trifle longer and more slender 

 and the eyes are on dark-coloured in'&te^dL oi pale prominences. 



The animal of both forms have the habit of keeping the 

 tentacles continually moving about very actively, whilst on the 

 contrary, H. ventrosa keeps them much more quiescent Tne 

 tentacles of both are also longer, more tapering and finer at the 

 points than those of H. ventrosa. The latter has the habit of 

 swimming inverted at the surface of the water, a position scarcely 

 ever (as far as I have seen) assumed by H. similis or H.jenkinsi. 

 They seem to prefer crawling. The distinction between H. 

 jenkinsi and the tAvo other species may thus be summarized : — 

 It differs from H. ventrosa (i) in habit, (2) length and form of 

 tentacles, (3) colour of the foot and head, and (4) in the greater 

 size and more robust form of the shells. Of the latter, some 

 have simple rounded whorls, others exhibit more or less of an 

 epidermal carination towards the upper part of them (var. carinata 

 of Marshall). 



From H. similis it differs (i) in its greater length, higher 

 spire, (2) less pronounced or unchannelled suture, (3) slighter 

 umbilical chink, (4) its tentacles are perhaps the least trifle 

 longer, and (5) the eyes are set on blackish instead of pale 

 swellings. 



The name of Amnicola confusa has been given by Frauen- 

 feld * to the shell which English conchologists usually regard as 

 the Hydrobia similis (Draparnaud). He states that he has 

 examined the original example of Draparnaud's Cylostoina simile 

 in the Imperial Museum at Vienna and finds that it belongs to 

 Bithynia, having the concentric operculum of that genus. 



As I see no just reason for doubting the correctness of 

 Frauenfeld's statement, it appears to me that we are no longer 

 justified in retaining the name of similis for this British species, 

 but must employ that suggested by Frauenfeld, as has already 

 been done by Paladilhe.f 



* Verhandl. Zool. Botan. Gesell. Wien., vol. xii, p. 1150 ; vol. xiii, 

 p. 1029 ; vol. xiv., p. 647. 



t Rev. Mag. Zool., 1S67, p. 47 ; 1S69, p. 229 nole. 



J 



