204 MANUAL OF THE MOLLUSCA. 
The development of the air-breathers goes on within the shell, 
and has been traced by Van Beneden, Gegenbaur, and others 
in Limax, Veronicella, Vitrina, Bulimus, and Helix. 
The shell of the gasteropods is usually spiral, and univalye ; 
more rarely tubular, or conical, and in one genus it is multivalve. 
The following are its principal modifications :— 
A. Regularly spiral, 
a. elongated or turreted; terebra, turritella. 
b. cylindrical; megaspira, pupa. 
c. short; buccinum. 
d. globular; natica, helix. 
e. depressed ; solarium. 
f. discoidal ; planorbis. 
g. conyolute; aperture as long as the shell; cypreea, Gullu. 
h. fusiform; tapering to each end, like fusus. 
z. trochiform ; conical, with a flat base, like trochus. 
k. turbinated ; conical, with a round base, like turbo. 
1. few-whorled ; Helix hemastoma. Pl. XII., Fig. 1. 
m. many-whorled ; Helix polygyrata. Pl. XII., Fig. 2.- 
n. ear-shaped; haliotis. 
B. Irregularly spiral; siliquaria, vermetus. 
C. Tubular; dentalium. 
D. Shield-shaped ; wmbrella, parmophorus. 
E. Boat-shaped; navicella. 
F. Conical or limpet-shaped ; patella. 
G. Multivalve and imbricated; chiton. 
The only symmetrical shells are those of carinaria, atlanta, 
dentalium, and the limpets.* 
Nearly all the spiral shells are dextral, or right-handed; a 
few are constantly sinistral, like clausilia ; reversed varieties of 
many shells, both dextral and sinistral, have been met with. 
The cavity of the shell is a single conical or spiral chamber ; 
no gasteropod has a multilocular shell like the nautilus, but 
spurious chambers are formed by particular species, such as 
Triton corrugatus (Fig. 69), and Huomphalus pentangulatus ; or 
under special circumstances, as when the upper part of the spire 
is destroyed. 
Some spiral shells are complete tubes, with the whorls sepa- 
rate, or scarcely in contact, as scalaria, cyclostoma, and valvata ; 
* The curve of the spiral shells and their opercula and also of the Nautilus, is a 
logarithmic spiral; so that to each particular species may be annexed a number 
indicating the ratio of the geometrical progression of the dimensions of its whorls 
Rev. H. Moseley, “On geometrical Forms of Turbinated and Discoid Shells.”—PAil, 
Trans, Lond. 1838. Pt. 2, p. 851. 
