204 MANUAL OF THE MOLLTJSCA. 



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

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

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



The sKell of the gasteropoda 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 ; terehra, turritella, 

 h. cylindrical ; megaspira, p>upa, 



c. short; huccinum. 



d. globular ; natica, helix. 



e. depressed; solarium. 



f. discoidal ; planorhis. 



g. convolute ; aperture as long as the shell ; cyproea, hulla. 

 li. fusiform; tapering to each end, liko/wsMS. 



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

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

 I. few-whorled ; Helix hcemastoma. PL XII., Eig. 1. 

 m. many-whorled ; Helix polygyrata. PI. XII., Fig. 2. 

 n. ear-shaped; haliotis. 



B. Irregularly spiral ; siliquaria, vermetus. 



C. Tubular; dentalium. 



D. Shield-shaj)ed ; umhixUa, parmophorus. 



E. Boat-shaped; navicella. 



F. Conical or limj)et-shaped ; patella. 



G. Multivalve and imbricated ; chiton. 



The only symmetrical shells are those of carinaria, atlanfa, 

 dentalium, and the limjoets.* 



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 

 ej^urious chambers are formed by particular species, such as 

 Triton corrugatus (Fig. 69), ^ndi Euomphalus pentangulatus ; or 

 ■under special circumstances, as when the ui^per part of the spiro 

 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 sheila 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 

 Eev. H. Moseley, " On geometrical Forms of Turbinated and Discoid Shells."— PAii. 

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



