204 MANUAL OP THE MOLLTJSCA, 



The development of the air-breathers goes on within the shell, 

 and has been traced by Van Beneden, Gegenbaur, and others 

 in Limax, Veronicella, Yitrina, Bulimns, and Helix. 



The shell of the gasteropods is usually spiral, and univalve ; 

 more rarely tubular, or conical, and in one genus it is multivalve. 

 The following are its principal modifications : — 



A. Eegularly spiral, 



a. elongated or turreted ; ferehra, turritella. 

 h. cylindrical ; megaspira, pupa, 



c. short; huccinum. 



d. globular ; natica, helix. 



e. depressed; solarium. 



f. discoidal ; plaiiorhis. 



g. convolute ; aperture as long as the shell ; cyprcea, bulla, 

 h. fusiform ; tapering to each end, like fusus. 



i. trochiform ; conical, with a flat base, like trochus. 

 h. turbinated ; conical, with a round base, like turbo. 



I. few-whorled ; Helix hcBmastoma. PI. XII., Fig. 1. 

 m. many-whorled ; Helix poly gy rata. PI. XII., Fig. 2, 

 n, ear-shaped; haliotis. 



B. Irregularly spiral ; siliquaria, vermetus. 



C. Tubular; deiitalium. 



D. Shield-shaped ; umbrella, 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, 

 denialium, 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. Euomphalus 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."— PAii. 

 Trans. Lond. 1838. Pt. 2, p. 351. 



