424 SUPPLEMENT 
resisting, and perhaps even altogether mobile, instead of 
being soft and flexible, as in the ordinary bivalves. All this 
might lead us to form a distinct order or perhaps even a par- 
ticular class of the terebratule. 
The definition of this genus may be thus summed up. 
Body oval, or rounded, compressed, horizontal, provided with 
a double pair of gills, free, resistent, extensible, turned in the 
form of a double comb, contained in a regular, dorso-ventral, 
equilateral, and inequivalve shell ; the upper valve in general 
longer, more gibbous, and lengthened into a top pierced with 
a hole, or emarginated ; the lower, shorter, more flat, provided 
internally with a system of support, or a very diversiform 
apophysis; the hinge ginglymoid, composed of two tubercles, 
or teeth, more or less separated, for each valve; no ligament ; 
muscular impressions not apparent. 
The terebratule, which are so extremely abundant in the 
fossil state, have as yet been found but in a small number in 
the living, probably because they remain fixed at tolerable 
depths, to immoveable bodies, and more especially to rocks 
that are always submerged. Accordingly, we know scarcely 
any thing concerning their manners, and their organization, 
but very imperfectly. We know, however, that some species 
of them exist in all seas. They are, in fact, to be found at 
the most remote points of the two hemispheres; that is, in the 
Norwegian seas, and those of New Holland, as well as in the 
seas of the warmest latitudes. Their habit of living fixed to 
rocks, and as it would seem, to other bodies, will account for 
the number of fossils to be found in this group of organized 
bodies. 
The last tribe of this class which we shall notice is that of 
ORBICULA. ‘This was established by M. de Lamarck, on a 
small shell of the northern seas, of which Miiller, and subse- 
quently Gmelin, made a patella ; because, not having observed 
the adherent valve, they took it for a univalve shell. The 
