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Summary.
The Mollusca collected by Messrs. Lucas and Hodgkin in the six lakes of New Zealand belong to nine genera, and these are represented by twelve species and five subspecies. As new we find one subspecies of Diplodon menziesi, one species of Corneocyclas, and one species of Gundlachia. Am interesting addition to our fauna is Diplodonta lessoni, Küster.
From Lake Rotoiti no specimens of Potamopyrgus badia, Gould, and Melanopsis trifasciata, Gray, were obtained, but it is almost certain that Potamopyrgus corolla, subsp. salleana, was taken for P. badia, and I never have seen an example of Melanopsis trifasciata from Lake Rotoiti, though I tried hard to get some. In the Lake of Tiberiade, in Palestine, Melanopsis has been found at a depth of about 150 ft.
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From Lake Wakatipu Planorbis corinna and Isidora antipodea were not represented.
All the Mollusca living in deeper water have much lighter colours, and the test is much thinner and more fragile. With one exception (Corneocyclas hodgkini) there is no real deep-lake fauna in these lakes, such as it is known from the subalpine lakes in Switzerland, &c., and this may partly be explained by the poverty of our lakes in molluscan life. Limnæa is absent, its place being taken by Amphipeplea, and it is interesting to find this genus represented by one species only.
The littoral fauna, according to Forel's investigations,* extends to 50–75 ft., and it is therefore evident that many of our known species can live in deeper water without undergoing any great structural change. Generally Unionidœ are not found in very deep water. However, Anodonta ponderosa, Pfr., was obtained in 33 ft. in Lake Tschaldyr, in Armenia, by Dr. Brandt, and five species of Unio were dredged in 150–300 ft. in Lake Tiberiade, in Palestine, by Lortet.
Sphœrium and Isidora have, as far as I know, not been known to live in deeper water, and of Potamopyrgus it is for the first time we get any accurate knowledge of the depths of water in which it may be found living.
The deep-lake molluscan fauna of New Zealand, as far as Messrs. Lucas and Hodgkin's investigations go, may be considered to be composed of the following species:—
(1.) Diplodon menziesi, Gray.
(2.) " " subsp. lucasi, Suter.
(3.) Sphærium novæ-zelandiœ, Deshayes.
(4.) Corneocyclas novozeelandica, Prime.
(5.) " hodgkini, Suter.
(6.) Potamopyrgus corolla, Gould.
(7.) " " subsp. salleana, Fischer.
(8.) Potamopyrgus badia, Gould.
(9.) Amphipeplea arguta, Hutton.
(10.) Isidora lirata, Tenison-Woods.
[Footnote] * Dr. F. A. Forel, “La Faune profonde des Lacs Suisses,” 1885.