460 
D? L. BOOS 
placed far enough in the system to regard them fairly different. I scar¬ 
cely fail to hit the mark by looking on the two species as having 
come from a common ancestor, or, putting it more correctly, — N. dana- 
bialis seems to descend from N. Prevostiana , the slight deviations from 
the latter being due to adaptation. The remarkable fact that it has 
attained to a larger size, finds its analogue in the mentioned example 
of the Neritinae of the Tátra Mountains. It is beyond our reach at 
present to find, in absence of experimental proofs, the agencies by 
which this increase has been brought about. I only beg to refer to 
Mr. Hazay’s interpretation , 1 who, on finding the same to be the case 
with forms of Bythinia tentaculata living in waters of different degree 
of temperature, attributed this change to the less amount of carbonic 
acid contained in cold water. 
Over and above what I have already said of the descensional 
relationships existing between N. Prevostiana and N. danubialis, I must 
additionally remark that questions like this are, naturally enough uncap- 
able of being solved through the knowdedge of the peculiarities of the 
shell alone. Final decision cannot be made until after the publication 
of the anatomical researches at hand. 
In this connection I must briefly touch upon the allied species, too, 
that live on wTiolly isolated spots separated by hundreds of miles from 
one another. N. peloponnesia lives in Greece, N. meridionalis in Sicily, 
N. sardoa in Sardinia, N. baetica in Spain and N. numidica in Algiers,-- 
yet they are so alike that they were more than once regarded as one 
and the same species, even by such authors of eminence as Deshaves, 
Philippi etc., — though Martens, the monographer on Neritinae, still 
advocates their being regarded as independent species on account of 
the persistency of the differences in form and, especially, in marking. — 
It is upon the strength of this declaration that recent investigations 
tend to similar results. 
It has repeatedly been emphasised by several conchologists that 
N. Prevostiana stands very near N. meridionalis , and Clessin 2 remarks 
that the two species reveal such a high degree of resemblance that it 
wants but a little for him to regard them identical. The only difference 
between the two of iY. meridionalis being somewhat more elongated 
in the direction of the greater diameter, is so insignificant that he is 
kept from uniting them by the long distance only, at which the one 
lives from the other. 
1 . Mollusken -Fauna von Budapest. 11. Tbei], p. 44. 
2 Fauna Öst.-Ung. Monarchie, p. 699, 
