256 SUPPLEMENT 
Navicular, some shells which, when turned on the back, 
with the aperture upwards, have a certain resemblance to a 
small boat, as the argonauta. 
Pyriform, when one of the extremities is thick or swelled, 
and rounded, and the other pointed in the form of a tail—for 
example, the pyrula. 
Conical, when one of the extremities, being widened, is as 
it were cut squarely, the other being pointed and forming the 
summit. When it is the summit or head of the shell itself 
which forms the summit of the cone, the shell is named tur- 
binated, as in the trochi; and it is called conical or convoid, 
when, on the contrary, the summit of the cone is at the ante- 
rior part of the aperture, as in the coni properly so called. 
Cylindrical, when the shell is elongated, and of a breadth 
or bigness pretty nearly similar, both in front and back. Such 
are most of the involuted shells, as the olives, &c. 
Fusiform, those which, swelled in the middle, are pointed 
at the two extremities—for example, the fusi. 
Turriculated, or turreted, those which are very much elon- 
gated; that is to say, whose longitudinal diameter is much 
longer than the transverse, which depends upon the manner 
in which the spire is formed—for example, the turritelle. 
6. The univalve shells may finally be considered in the 
relation of the distinction of each of their parts. 
A univalve shell may be conceived always to have a sum- 
mit or point where it has begun, a base which is its actual 
termination, and an intermediate body, with a cavity sometimes 
almost imperceptible, in the case where it is extremely de- 
pressed or altogether flat, and then it has really many relations 
with one valve of a bivalve shell. It is altogether the reverse 
in the tubular or tubiform shells, which much resemble the 
calcareous tubes of certain setipodes. 
But before proceeding farther, it may not be superfluous to 
explain the position in which conchologists place the univalve 
