46 The Irish Naturalist. March, 



be attending to the subject of spires in this group — making 

 experiments in spires. Hence the negative and positive 

 variation is directed niainl}' to the .spire ; some forms have it 

 exceptional!}' high, ^vhile in others it is exceptionally low. 

 In the same wa}^ in the genus Vallonia we find costation 

 strongh' developed, and also its complete absence ; in the 

 radula of the difficult " glabra " group of Vitreae we find both 

 unusual shortening and unusual lengthening of the central 

 mesocone; other examples will occur to every student of the 

 Mollusca, in which a new and pronounced character is 

 accompanied in the same group of forms by its inversion. 

 The bearing of this upon the Mendelian hypothesis will be 

 evident. It may also be noted that if there is some such 

 affinity between LimncEa glabra and the alpine Limnaeas, 

 they may be in some sense alternative to one another, and 

 this would give a meaning to the recorded non-occurrence of 

 Z. glabra in the district where these latter are found. 



At the suggestion of Mr. B. B. Woodward, I propose for 

 the Lough Nagarriva form the name of Limncea p7'(stcnuis^ in 

 allusion to its remarkabl}^ thin shell. 



LlMN^A PR^.TENUIS, n.Sp. 

 {Irish Naturalist, vol. xvi., pp. 286-8, pi. 36.) 



L. involutes proxima, testa ovata conglobata terete 

 neritiformi, subdiaphana, colore corneo, t.enuissima 

 (o"02 ad 0.03 mm.), striis notaia spiralibus minutissimis 

 exacte dispositis ; anfractibus quattuor rapide crescenti- 

 bus ; spira appressa parum eminente, interdum erosa ; 

 columella obliqua leviter contorta, subtus vix reflexa ; 

 apertura ovali subpyriformi, altitudinem gyri penultimi 

 psene duplicante. H. 9-10; d. 8-9 mm. Hab. : I^ough 

 Nagarriva. 



The type specimens are in the collection of Mr. A. S. 

 Kennard. The extreme tenuity of the shell is further 

 exemplified by the fact that three specimens weighed less 

 than 3 milligrammes. It appears evident that this is not a 

 case of irregular or pathological variation, for all the animals 

 examined were " well set up," and appeared to be in every 

 way healthy examples of a fixed and uniform type. The 



