50 MALACOZOA. GASTEROPODA. PECTINIBEANCHIATA. 



with six gradually increasing volutions, the last with an 

 acute angle or keel along the lower margin of the disk, 

 the aperture roundish, somewhat rhombic* Vurtex> a 

 whirlpool, circular movement, or coil. 



A. P. Vortex crassulus. Dim. of crdssus, fat or thick. 



B. P. Vortex Spirorbis. S'pira, a whorl ; orbis, a ball. 



2. Plantrlis contortus. Shell orbicular, depressed, 

 widely and slightly concave above, with six nearly equal, 

 narrow, distinctly striated volutions, the last rounded, 

 leaving a wide and deep cavity beneath ; mouth narrow, 

 semiluuar. Contortus, twisted together or coiled. 



3. Planorbis dlbus. Shell orbicular, extremely de- 

 pressed, widely concave above and beneath, with the 

 volutions longitudinally sulcato-striate, transversely striu- 

 late, and hispid. Allus, white. 



4. Plandrbis imbricdtus. Shell orbicular, extremely 

 depressed, widely and very deeply concave beneath, 

 slightly concave in the middle above, with the volutions 

 obliquely sulcato-striate. Imbriedtus, 



GENUS 4. ANCYLUS. Animal conoidal, with two de- 

 pressed, elongated, triangular, obtuse, tentacula. Shell 

 elliptical, conoidal, with the point curved backwards, 

 and nearer the posterior end. Ay/cuX^, a kind of cup. 

 GeoiFroy. 



1. Ancylus fluviatilis. Oval, conoidal, striulate lon- 

 gitudinally and concentrically. Fluvidtllis, residing 

 in rivers. 



ORDER II. GASTEROPODA PECTINIBRANCHIATA. 



Respiratory apparatus a branchial cavity on the back, 

 containing two fimbriated branchiae, and communicating 

 externally by a large aperture between the edge of the 

 mantle and the body. Sexes separated. 



Chismobranchidta, Blainville. Trachelipoda, Lamarck. 

 Pectinibranchidta, Cuvier. Pecten-, a comb ; Branchiae* 



