430 MOLLUSCA. 
I may mention here that in the fresh waters of Mexico and Central America, as in 
those of many other countries, spirally twisted cases or shell-like objects are to be 
found which might. be easily mistaken for true shells of the genera Valvata or 
Cochliopa ; but which, at first sight, differ from those forms in their coarsely tubercu- 
lated external structure, due to being composed of small agglutinated fragments of 
stones, the work of insect-larve of the genus Helicopsyche, family Phryganeide. The 
Berlin Museum possesses specimens of such cases from the Rio Colipa in Mexico, 
collected by Deppe, and formerly named Paludina agglutinans; also others from 
Orizaba, collected by Berendt, and some from the Rio Torres, San Francisco, 
Costa Rica, collected by H. Pittier; Salvin also found a similar case at Coban, 
Guatemala. They are depressed-conoidal, umbilicated, 5 millim. in diameter and 
2-3 millim. in height; the spiral whorls are dextral, as usual in shells, whereas there 
are some similarly formed cases of terrestrial larve of insects (fam. Psychide, 
gen. Cochlophora), composed of sand or mud, which are sinistral. (See v. Martens, in 
Sitzungsberichte d. Gesellsch. naturforsch. Freunde in Berlin, 1891, p. 83.) 
AMNICOLA. 
Amnicola, Gould & Haldeman, Monogr. Limniades, Suppl. p. 3 (1840); Binney, Land and 
Freshw. Shells of N. Am. iii. p. 80; Fischer & Crosse, Miss. Scient. Mex., Mollusca, ii. 
p- 256. 
Shell of small size, thin, ovate or oblong, of 4—6 whorls, generally smooth, in some 
species keeled and spinous, mostly perforated ; aperture ovate, angulated above, rounded 
beneath, peristome continuous, simple. 
Operculum paucispiral, horny. 
Tentacles long, filiform ; eyes at their outer base. 
Central tooth of the radula broadly triangular, with numerous cusps, and with several 
denticules on its plane near the base; lateral and marginal teeth long, somewhat 
curved, the lateral (intermediate) one with several distinct cusps, the two marginal 
teeth finely crenulated. 
I adopt here the classification of Fischer and Crosse, who place all the small 
Hydrobioid shells of Mexico and Central America, except 7; ryonia, in this genus, not 
because I am convinced that they are all congeneric with the North-American 
A, limosa, Say, but because I cannot offer a more satisfactory arrangement. For the 
same reason, it is useless to discuss the geographical distribution of this genus, as it is 
extremely doubtful. 
1. Amnicola petenensis. 
Paludina petenensis, Morelet, Test. Noviss. il. p. 21 (1851) ‘. 
Amnicola petenensis, v. Frauenfeld, Verh. zool.-bot. Ges. Wien, xiv. p. 6385 (1864)°; Fisch. & 
Crosse, Miss. Scient. Mex., Mollusca, ii. p. 262, t. 50. figg. 8, 3.a, 5°. 
Hab. N. Guaremaua: Lake Itza, Province of Peten (Morelet 1-3), 
