20 SYNOPTIC KEY 



6(5, 7) Marine. Adjacent filaments either with interlocking 

 ciHa or with interfilamentar junctions. Foot usually 

 only slightly developed. Usually only one adductor 

 muscle present. The two valves of the shell often 

 unequal. (Pratt, 573.) 



Order Pseudolamellibranchiata 



7(5,6) Marine or fresh-water forms. Interfilamentar and 

 interlamellar junctions well developed and numerous, 

 making the gill present a continuous surface. Includes 

 most pelecypods. (Pratt, 575; W. and W., 994; 

 Baker, 44.) Order Eulamellibranchiata 



Class GASTROPODA 



1(2) Fresh-water or terrestrial forms. Shell present, 

 external, or internal or absent. Mantle cavity almost 

 always modified to form a lung for breathing air; 

 opening of the mantle cavity small. Adults without 

 an operculum. (Pratt, 506; W. and W., 977; 

 Baker, 138.) Order Pulmonata 



2(1) Gills present; mantle cavity not modified to form a 

 lung; shells in fresh- water species usually closed by 

 an operculum. 3 



3(4) In all fresh- water forms and in many others an opercu- 

 lum present closing more or less completely the shell 

 mouth when the animal is retracted. Shell almost 

 always present. Mantle cavity situated on the anterior 

 side of the visceral mass. Usually only one gill, which 

 is situated anterior to the heart. (Pratt, 528; Baker, 

 319; W. and W., 986.) Order Prosobranchiata 



4(3) Mostly marine forms with shell spiral and external, 

 or reduced and internal or absent. Mantle cavity 

 absent or present; gills either a true ctenidia or in 

 form of adaptive gills (as in sea slugs). Ctenidia 

 when present posterior in position. Fresh-water 

 forms few. (Pratt, 495; W. and W., 994.) 



Order Opistobranchiata 



