Forms, Composition, and Structure of Shells. 337 
an accurate notion of what is a foliated and what a compact 
shell. 
Univalves are still more numerous than bivalves, and more 
diversified and beautiful in their forms and colouring. Some 
are simply conical (Patélla Zin.), either entire or perforated 
on the summit, or slit at the margin; some are tube-like 
(Dentalium) ; some bear a similitude to the human ear (Hali- 
otis Lin.) ; but by far the greater number are spiral, the 
whorls being contorted in a thousand pleasing ways. ‘Thus 
the Plandrbis and some others have them rolled upon one 
another without being raised, so that the last whorl is on the 
outside, and forms the margin. ‘These flattened shells are 
said to be discoid. In Helix, Nerita, and Dolium, the last 
whorl is so disproportionally large to those which constitute 
the spire, that the shell assumes a globular shape ; in 7rochus 
it is a regular pyramid ; in Térebra and Turritélla the whorls 
are so numerous and elongated that the shell resembles a 
turret, and is hence said to be turreted or turriculate. But 
between these extremes there are many intermediate forms, 
and the spire in general is a more or less acute oblique cone, 
tapered from the base, or with its greatest diameter towards 
the middle. There is, however, a large and beautiful tribe 
of univalve shells, which are denominated convolute; for the 
whorls, which are small segments of large circles, are wrap- 
ped round the pillar, and, the one rising a little above the 
other, embrace or enclose the preceding ones. Hence it 
results, that the aperture of a convolute shell must be parallel 
to its length. The cowries (Cypre‘a), volutes, and cones are 
examples of this kind of formation, and in elegance of form 
and brilliancy of colouring they exceed all others of the class. 
The Nattilus and Argonatita are, on the contrary, revolute 
shells; that is to say, their whorls are twisted backwards into 
a spire, which is contained within the outer whorl. * 
Revolute shells, in common with many discoid ones, differ 
in a very remarkable character from other univalves ; for their 
cavity is divided by transverse partitions, more or less com- 
plete, into many chambers ; and hence they are called cham- 
bered, or, more commonly, multilocular, shells. ‘The recent 
shells of this description are few in species, nor are the species 
very numerous in individuals; but the fossil kinds are many 
and abundant, and some of them have been of a size so great 
that they were not unworthy to play their part with the cro- 
codiles, the Ichthyosatri and Plesiosatiri of a former world. 
* The reader will find these forms delineated in some beautiful cuts 
which illustrate a paper signed Conchilla, in Vol. L. p. 25. et seq. 
