UNIO.—ANODONTA. 523 
may be nearer to U. aztecorum, Phil. If it is, indeed, from the Lake of Chalco, it is 
the only species of Unio found in the waters on the central plateau of Mexico which 
do not drain to the Pacific or Atlantic. 
Unio sp., aff. parvo. . 
Unio (Lampsilis) sp., an parvus ?, v. Mart. Malak, Blatt. xii. p- 60 (1865). 
In the Berlin Museum there is a broken example of a little Unio nearly allied to: 
U. (Micromya) parvus, Barn., with a considerable swelling in the hinder part of the 
shell. It was given by C. Uhde, with the label “Vera Cruz, River Medellin.” As 
the specimen is too imperfect to be figured, and a confusion of the labels during the 
transport of the collection from Heidelberg to Berlin may have taken place, I refrain 
from describing it at present. 
ANODONTA. 
Anodontites, Bruguiére, Choise de Mémoires sur divers objets d’Hist. Nat. (1792) ; Cuvier (1798) ; 
Poiret (1801). | 
Anodonta, Lamarck, Prodrome d’une Classification de Coquilles (1799); Draparnaud (1801) ; 
Cuvier (1817), &c. 
Resembling the genus Unio in most respects, but with the hinge simple, without teeth; shell generally rather 
thin. Living chiefly in quiet stagnant water. 
For a more complete account of the history and of the systematic and anatomical 
characters of this genus, see Fischer and Crosse (Miss. Scient. Mex., Moll. ii. pp. 509 
et seq.). As in Unio, the sexes are mostly separated, and the female individual some- 
times recognizable by the greater convexity of the shell, due to the development of the 
eggs within the gills *. 
Geographical distribution :—Widely spread on all the continents, but scarcely repre- 
sented in Tropical Africa. Northwards, it extends in Europe and America somewhat 
further than Unio—A. frigida, Drouet, in Sweden, beyond 62° N. lat.; another species 
in the territory of Ob and Yenisei, up to 62°-63°; A. kennicotti and A. simpsoniana, 
Lea, in the Great Slave Lake, beyond 60°; A. yukonensis, Lea, about 60°-65°,—but not 
so far as the smaller freshwater snails (Limneide), nor, in Europe, as Margaritana 
margaritifera, L. In Switzerland and the Tyrol it is found in small mountain-lakes, 
ascending somewhat higher than the genus Unio. On the central plateau of Mexico we 
know as yet three localities only for Anodonta—the Lake of Chalco (2276 metr.), the 
environs of the city of Mexico (2270 metr.), and the Lake of Chapala (1700 metr.). 
Fischer and Crosse have grouped the Mexican and Central-American forms of this. 
genus under five sections, giving to each of them a new subgeneric name, without regard 
to the species from other parts of the world. They do not mention the shape of the 
* Further observations on the development of the egg in the Mexican and Central-American species are 
66* 
much desired. 
