TESTACEA, in the Linnaean system of natural history, the third order of vermes. This order comprehends all shell-fish arranged by Linnaeus under 30 genera. Shell-fish are animals with a soft body, covered by or inclosed in a firm, hard, and as it were stony habitation, composed, according to their three separate orders, 1st. Of many parts which are ranged under the name of multivalves; 2d. Of two parts which are called valves; 3d. Of one part or piece only, which we call univalves. These parts, pieces, or valves, are more or less moveable at the animal's pleasure. The animals included in these hard habitations have most of them the characters of one or other of the genera vermium, and might be reduced under the same genera with the mollusca: but as these characters are few, and the shells very numerous, and different in their form and structure, it will tend more to make this part of natural history easy, to arrange the subjects according to the distinctions of the shells themselves.
There is this further consideration in favour of this arrangement, viz. that the animals themselves are rarely seen, and never can be preserved in cabinets; whereas the shells make a figure in them, and great numbers have been met with empty of the animal.