50 MALACOZOA. GASTEROPODA. PECTINIBILANCH1ATA. 



with six gradually increasing volutions, the last with an 

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

 the aperitive roundish, somewhat rhombic* Vortex, a 

 whirlpool, circular movement, or coil. 



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



B. P. Voriex Spiroruts. Spiraea whorl \ drbis^ ball. 



2. Planorbis conturtus. — Shell orbicular, depressed, 

 widely and slightly concave above, with six aea rly equal, 

 narrow, distinctly striated Volutions, the last rounded, 

 leaving a wide and deep cavity beneath ; mouth narrow, 

 semilunar. Contortus, twisted together or coiled. 



3. Planorbis dlbus. — Shell orbicular, extremely de- 

 pressed, widely concave above and beneath, with the 

 volutions longitudinally snlcalo-sti late, transversely stimu- 

 late, and hispid. Alius, white. 



4. Planorbis imbriedtus. — 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. Anctlus. — Animal conoidal, with two de- 

 pressed, elongated, triangular, obtuse, tentacula. Shell 

 elliptical, conoidal, with the point curved backwards, 

 and nearer the posterior end. AyicvXr), a kind of cup. 

 Geoffroy. 



1. Ancylus fluvidtilis. — Oval, conoidal, striulate lon- 

 gitudinally and concentrically. Fluvidtilis, 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 ; Brdnchite, 

 gills. 



