122 The Irish Naturalist. Aug.-Sept., 



of Lough " H ") and flows in a north-easterly direction 

 into the Kerry River. The other stream from Lough 

 " H '' and also that from Lough " F " flow into Lough 

 " C," and the stream from the latter after receiving the 

 rivulets from the two isolated Loughs " B " and " A " 

 flows into Barley Lake, the river from which, the Owena- 

 cahina, effects a junction with the Kerry River in the 

 valley to form the main Glengarriff River. 



It will be seen by reference to my list given below that 

 the three " species " L. involuta, L. praetenuis, and L. 

 pereger, are found on each side of the main ridge, and also 

 that almost every lake of the Caha series is connected with 

 at least one other. Having regard to the shortness of the 

 distance traversed by these streams, and their size, it is 

 quite certain that in rainy weather a fair number of molluscs 

 must be washed from one lake into another, yet in none 

 of them can two kinds be found together. This phenomenon 

 is all the more striking as egg-capsules, which appear to 

 be chiefl}' deposited on the loose fragments of dead sheep- 

 grass mentioned above, must be washed from one pool to 

 another with the slightest freshet, as the pieces of grass 

 were running freely out of several of the pools on the day 

 of my first visit, when we had had rain two days previously. 

 It is idle to suppose that conditions can be so widely 

 different in the lakes that it is physically impossible for one 

 species to exist in a locality which supports abundantly 

 members of another, yet to some such theory must anyone 

 be driven who has examined the plateau unless he take 

 the simple solution that L. involuta and L. praetenuis are 

 " syntonic " forms of L. pereger, in which case the difficulty 

 disappears at once. In a syntonic form, as defined by 

 Messrs. Kennard and Woodward ,i varietal characters are 

 not inherited but remain constant so long as environment 

 is unchanged, a resumption of normal conditions causing 

 reversion to type. Thus L. praetenuis, if a syntonic form 

 of L. pereger, would, on coming into a lough containing 

 L. pereger, produce nothing but L. pereger. 



1 " The Post-Pliocene Non-Marine Mollusca of Ireland." Proceedings 

 of the Geologists' Association, vol. xxviii., p. 112, part 3, 1917. 



