800 PROF. G. C. BOURNE ON TEE 



fully realised in some species* than in others, to 'a reduction of the 

 stalk and articular cavity, this reduction being shown to a slight 

 degree in Aloadia, to a marked degree in Lncidella. 



In all the Helicinidie there is a tendency to the reduction of 

 the central and admedian teeth : this tendency is shown in a 

 marked degree in Lucidella and Orohophana, but must have 

 reached its present degree independently in these two genera. 

 The reduction is carried to an extreme degree in Hydrocena, in 

 which the second and third admedians have disappeared ; the 

 median and first admedian are present, but in a rudimentary 

 condition, and the laterals are reduced to mere rods of no great 

 size. But I am disposed to think that the Hydrocenidfe must 

 have branched oif from the Iferitoid stock iiulependently of the 

 Helicinidse. They retain many primitive features, as Thiele has 

 shown, among others the process of the opei'culum which is quite 

 Neritoid in character, and their geographical distribution favours 

 this view. Hydrocena is confined to the marine littoral of 

 Dalmatia ; Georissa lives at considerable altitudes on the Khasi 

 Hills in India. It is by no means improbable tliat pulmonate 

 forms may have been developed more than once from such 

 animals as the 'Neritidse, which show a predilection for migrating 

 as far as possible out of the watei", and for the rest of it, the most 

 that can be said in favour of uniting the Hj^drocenidse with the 

 Helicinidfe is that both display strong Neritoid affinities. 



The main result of my researches is to show that in such a 

 limited group as the Helicinidse the systematists are justified in 

 their methods. The visceral anatomy of all the forms that I 

 have examined is strikingly similar, and where deviations occur 

 they are contradictory and of uncertain value. Tlie Helicinidee 

 appear to have inherited an organization with marked Neritoid 

 characteristics, and to have maintained it, with little or no 

 change. Presumably that orgaiiization is weTl adapted to the 

 somewhat narrow i-ange of the conditions of their existence, and 

 any deviation fi-om it has been checked by the action of natural 

 selection. But there are a thousand deviations, in all directions, 

 among characters which cannot by any stretch of the imagination 

 be claimed to be of any importance in the struggle for existence. 

 Such characters are the texture and coloration of the shell ; the 

 shape of the aperture ; the extent and distinctness of the basal 

 callus ; the presence or absence of folds at the aperture of the 

 shell ; the presence or absence of a minute notcli, such as occurs 

 in Alcadia ; the arrangement of the growth-lines on the opei'- 

 culum. It might be said that the operculum is an important 

 protective organ and therefore eminently susceptible to the action 

 of natural selection. But its function is simply to close the 

 aperture of the shell, and this it does equally efficiently in all 

 the species that I liave examined, the numbei- of these being 

 much larger than the few available for anatomical study. As 

 long as the operculum performs this function efficiently minute 

 characters, such as the greater or less distance of its nucleus from 



