VOL. LXr.] PHILOSOPHICAL TRAKSACTIONS. 133 



pany, they hide themselves in their shells, and shut up the valves, which force 

 and distress attempt in vain to force open. 



Mr. M. discovered several different species of these animals in the waters of 

 Fridericksdal, one only of which is known to the naturalists. Mr. Baker, of 

 the Royal Society of London, is the first who mentions it ; he says, ♦ that the 

 insect swims very fast ; that it procures its nourishment by means of a whirlpool, 

 which it raises in the water by means of its arms ; that on meeting with a solid 

 body, it stops itself by means of its feet ; that on the slightest touch it shrinks 

 into its shell ; and lastly that it bears much resemblance to a bivalve shell-fish.' 

 To this description he joins a figure, which though imperfect, represents the 

 insect. Linnaeus and GeofFroi call it the monoculus, and without taking notice 

 that Mr. Baker knew it already, they observe that its antennae are composed of 

 small white threads; and that the shell is oblong, smooth, and greyish, round on 

 one side, flat on the other, and nearly of the same size at each end. None of 

 the above-mentioned writers have had the satisfaction of inspecting the inhabitant 

 of the shell, which indeed is very difficult. Now as this insect bears a strong 

 likeness to the nev/ species, Mr. M. takes a view of both together. 



These minute bivalves are found adhering to confervas. That now described, 

 he found, in the autumn of 1768, at the botton) of a ditch of standing water. 

 The transparency of the shell gave him an opportunity of examining the inhabit- 

 ant ; and the examination cleared up the doubt he had about its species. The 

 new shell is a bivalve ; white, smooth, shining, and transparent, without the 

 least spot, hair, or down. Its figure is oblong, rounded at both ends, and the 

 hinge somewhat sinuated at the opening, and convex at the sides, in such a 

 manner as when seen out of water, it is very like the seeds of some plants; and 

 this is common to all the species of this genus. The substance is coriaceous, or 

 like hardened glue; thin and very brittle when dried. When seen by the mi- 

 croscope, some of them appear very like net-work. The valves are equal, a little 

 broader at one end than at the other, and somewhat flatted at the slope ; they 

 are not however more elevated at the opening than at the hinge, but rather the 

 contrary; for on the inside they show another edge, less elevated than that of 

 the outside, and which becomes less and less towards the hinge. 



The length of the shell is half a line, and its greater breadth above a quarter 

 of a line. That species mentioned by the above writers is 3 times longer when 

 at its full growth. It is hairy, though smooth to the naked eye, more indented 

 at the slopes where the valves are projecting, and more depressed towards the 

 hinge; it is opaque, and of a changeable colour. Some of these insacts are of a light, 

 and others of a dark green, marked with an oblique stripe lighter than the rest. 

 Some of them are bright, and others grey and dirty ; but the down with which 



