ON PTEROPODA. 317 
all subsequent zoologists have placed it where it should be, 
namely, in the neighbourhood of the Clio, as it stands in the 
text. 
The body of the most common hyalza, of the size of a 
tolerably middling nut, at least when it is contracted, is 
formed of two parts, separated by a very well marked 
contraction. One anterior, containing the head and a sort 
of thorax ; and the other posterior, which might be named 
abdomen. ‘The latter is always covered by a sort of shell 
of a rather singular form, and which might, and has been 
taken for a bivalve, the valves of which were united closely, 
or continued to the place of the hinge. The fact is, that 
it is rather a kind of sheath, very symmetrical, very much de- 
pressed, and in which the anterior aperture is prolonged into 
a very narrow emargination on each side. This sheath, very 
thin, although hard, of the colour of horn, and translucent, 
is pretty nearly square. The posterior edge, on which the 
two lamin are confounded, is divided more or less deeply 
into three points, the middle one of which, always longer, is 
pierced at its extremity. The lateral edges are straight and 
cleft, for a greater or less part of their length. As for the 
anterior edge, it presents the aperture of the sheath or shell, 
_ which is rather narrow and transverse. The upper part ad- 
vances much more than the lower, because the upper lamina, 
which is almost plane, with four sorts of keels, radiating from 
the middle point, is prolonged, forming in front, a sort of 
blunt apophysis. The under plate, on the contrary, is much 
more hollow, considerably more gibbous, and as it were 
hemispherical, and its anterior edge is rounded. 
This shell is completely uncovered, and is attached to the 
animal only by its middle or pierced point, to which the 
dorsal muscles, or those of the pillar are attached, and by the 
edges of its aperture, to which those of the mantle adhere. 
Although it really appertains only to the abdomen, it ap- 
