10 FOOT. 



other genera, as in Oliva, the mantle is prolonged into filiform 

 processes before and behind. 



The female Vermetus has the mantle cleft in the middle, ac- 

 cording to Lacaze-Duthiers, although there is no corresponding 

 cleft in the shell, and in Haliotis a similar mantle cleft impresses 

 a groove in the shell, in which are situated the row of holes 

 characteristic of the genus. The shell of Pleurotoma also has 

 a sinus corresponding to a cleft mantle. The cause of the 

 sutural sinus of the shell of the American fresh-water genus 

 Schizostoma is as yet unknown; it may be due to a similar cause 

 or it may be sexual. As the genus is restricted to the Coosa 

 River and its neighborhood, I am inclined to think that it is a 

 local disturbance of growth, especially as most of the species 

 could not be distinguished from corresponding forms of Gonio- 

 basis except by the lip notch or slit. 



Foot. 



The foot is a fleshy, expanded mass, attached to the under 

 side of the body, in front of the mantle by a peduncle. In the 

 heteropods the foot is divisible into three portions, termed, 

 respectively, propodium, mesopodium and metapodium ; but in 

 the typical gasteropods these three areas are blended in the sole, 

 although the metapodium is indicated by the fact of its support- 

 ing on its dorsal side the operculum. In Strombus (pi. 3, fig. 

 14), a transverse furrow separates the mesopodium from the 

 propodium, and the metapodium is covered downwards and in 

 front by the operculum. 



The peduncle of the foot is usually short and depressed", and 

 covers the under side of the body between the mantle collar and 

 mouth, the foot being expanded forward, but more extensively 

 backwards ; but in Strombus and its allies the operculum is long 

 and narrow, whilst the foot is slim and cylindrical. Haliotis, 

 Patella and Chiton have the foot, on the other hand, very much 

 expanded. Rapidity of motion appears to be in inverse ratio to 

 the size of the foot ; those genera in which this organ is enor- 

 mously developed, especially in those just cited, where it occu- 

 pies the entire ventral surface of the body, being slow in 

 movement. 



