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 (Patella Lin.), 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- 

 hiis Lin.) ; but by far the greater number are spiral, the 

 whorls being contorted in a thousand pleasing ways. Thus 

 the Planorbis 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, A^erita, and Dolium, the last 

 whorl is so disproportionally large to those which constitute 

 the spire, that the shell assumes a globular shape ; in Zrochus 

 it is a regular pyramid ; in Terebra and Turritella 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 (Cypr^^«), 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 iVautilus and Argonauta 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 Ichthyosauri and Plesiosauri of a former world. 



* The reader will find these forms delineated in some beautiful cuts 

 which illustrate a paper signed Conchilla, in Vol. I. p. 25. et seq. 



