498 On the American Slipper-Limpet. 



case it would be, on the whole, favoured by the tidal stream 

 {cf. Orton (7)). According to information kindly supplied by 

 the Hydrographical Department o£ the Admiralty, the tidal 

 stream runs from Orfordness to Harwich nearly parallel to the 

 coast S.W. on the flood, turns W.S.W. across the shoals into 

 Harwich Harbour, S. off the Naze, S.W. off Frinton, and 

 W. by N. off the Wallet into the Blackwater River. On 

 the ebb the above bearings are all approximately reversed. 

 Fuither, the ebb or N.E. going tide is slightly stronger than 

 the flood — a fact which would tend to favour the spread of 

 the larvse up the coast northwards. Dr. Fowler informs me 

 that the general normal drift (resultant of tide + wind + 

 current) is stiith-westerly. As Lo Bianco has pointed out (4), 

 the annual distribution of plankton is very much affected 

 by drift-currents (those produced by wind), so we cannot 

 afford to neglect this factor (see below). 



North of the Naze the Essex coast is indented by an irre- 

 gularly-shaped arm of the sea, several miles in area, and 

 consisting of sands uncovered at low tide, saltings and channels 

 of various width meandering among sand and mud-flats. 

 It would be extremely interesting to discover if Grepidula 

 fornicata has spread round this maze of channels and flats and 

 reached the Dovercourt coast by this route, or whether, as is 

 more probable, it is absent from this area, having been carried 

 across to the opposite coast direct by the tidal stream. 



In any case, whether Grepidula fornicata is extending its 

 range by natural transit or as a " stowaway " on coasting 

 .vessels and yachts, the alternatives are sufficiently interesting 

 to warrant discussion. It should be pointed out that, if 

 the first method is the one responsible, the progress must 

 be slow, as the balance in favour of the N.E. going ebb is 

 not very great, the extreme rates of the ebb and flow being 

 as follows * : — 



Knots. 



f Ebb (N.E.) 3 bours after H.W. Dover 08 to 1-9 



4 „ „ , 0-7 to 1-5 





Flood (S.W.) 4 hours before H.W. Dover . . 0-1 to 1-7 

 C. 3 „ „ „ 0-5 to 17 



According to Orton (5) the spawning-period of C. fornicata 

 extends from March to November, the veligers swimming 

 free for about two weeks (Conklin (6)). Larva; are thus 

 practically certain to be present in the plankton during April, 

 in which month, according to information kindly given me 



* At the Cork Light off Harwich. 



