SWAINSON. 577 



Every order is, of course, circular, after the fashion of the 

 primary ones. Thus, after rejecting all genera irreducible 

 to the system (for the bed of Procrustes is oftener used than 

 it ought to be in a well-ordered house), the Gasteropoda 

 have five tribes, viz. : 1, the Zoophaga of Lamarck ; 2, the 

 Phytophaga of the same author : 3, the Scutibranches ; 4, the 

 Cyclobranches ; and 5, the Tectibranches of Cuvier. And 

 that these tribes are natural divisions is presumptively 

 proved by the fact of their analogizing exactly with the 

 orders of the Mollusca, as thus : 



Tribes ofGaste- Analogical Characters. Or ers f the 



ropoda. Testacea. 



f Pre-eminently typical ; mantle formed > 



t into one or two long tubular siphons. } GASTEROPODA. 



PHYTOPHAGA. Siphons entirely wanting. DITHYRA. 



f Animal oval, greatly depressed; the \ 

 SCUTIBRANCHIA. -| branchia, in the typical divisions, [.NUDIBRANCHIA. 



|_ fringed, and placed on the back. j 



( Tentacula none ; body broad, oval, -, -n 

 CYCLOBRANCHIA. { onigciform> '}PARENCHYMATA. 



r Shell, when present, protecting only a^j 



TECTIBRANCHIA. < part of the body, concealed ; mantle I CEPHALOPODA. 

 {_ dilated into fin-like lobes. j 



Each tribe has again its five " leading divisions." Those 

 of the Zoophaga are: 1. The MURICID^:, or Murexes, 

 having the respiratory siphon in general very much deve- 

 loped, and its corresponding canal at the base of the shell 

 always straight. 2. The TURBINELLID^E, or Turnip-shells, 

 where the base of the shell is straight and lengthened, and 

 the pillar strongly plaited. In both these, the mantle of the 

 animal is never dilated, but is of ordinary dimensions, and 

 drawn back into the shell with the animal. 3. The VOLU- 

 TID^E, or Volutes, having the mantle much developed in the 

 typical species : the column of the shell is always marked by 

 regular and well-defined plaits, nearly the same as in the last ; 

 but the base of the aperture is obtuse, truncate, and notched. 

 4. The CYPR^EID^:, or Cowries, whose shells are without any 

 spire ; the last whorl enveloping all the others, as in the 

 Bullas. 5. The STROMBID.E, or Wing-shells, the only divi- 

 sion where the outer side or lip of the aperture is consider- 

 ably dilated. The first two of these are the typical and 

 sub-typical groups ; the latter three forming the aberrant 

 circle. The further analysis of the typical and sub-typical 

 gr oups is exhibited in the following diagram : 



p P 



