74 HELICID.E. 



ttt Palatal folds, one vertical, rest horizontal ; 



shell 7"5 mm shiroie)2sis. 



*"* No third fold present 7iagaensis. 



h. Horizontal fold below transverse parietal j)late 



long, joined to apertural rid^e pernerfe. 



c. Three short horizontal folds in front of transverse 



parietal plate, none below it refuga. 



B. Transverse parietal plate giving otf anteriorly below 



a short horizontal fold ; a long median and a long- 

 lower fold present, joined to apertural ridge. 



a. Palatal folds : all horizontal shanensis. 



h. Palatal folds : one oblique, rest horizontal leiophis. 



C. Transverse parietal plate giving off anteriorly below 



a short horizontal fold, two short free horizontal 

 folds above the latter, and a long one below joining 

 the apertural ridge hrahma. 



IIT. Section Endoplon, Gude. 

 Dextral. Palatal folds horizontal, oblique, or almost vertical. 

 Type, P. h-achyplecta. 

 Range. Burma, Tonkin. 



Shell flattened. 



n. Two vertical parietal plates brachyplecta. 



h. One vertical parietal plate with two denticles in 



front smithiana. 



W. Section Plectopylis, s. s. Qucle. 

 (Typical Section of Benson.) 



Sinistral. Shell flattened. Palatal armature : one vertical lamina 

 with three horizontal folds above, one below. 



Type, P. bensoni. 



Range. Burma. 



A. Two transverse parietal plates. 



a. Parietal plates parallel ; upper horizontal palatal 

 fold bisected. Shell less than 20 mm. 

 a. Median parietal fold truncate, not joined to 



apertural ridge ponsonbyi. 



(3. Median parietal fold not truncate, joined to 



apertural ridge lissochlamys. 



h. Parietal plates divergent ; upper horizontal palatal 



fold not bisected. Shell more than 20 mm magna. 



c. Anterior parietal plate giving off a long horizontal 

 fold above, and 

 a. A short one below, half the length of upper ; 



palatal folds in two series ivoodthorpei. 



/3. Lower fold one-quarter of the length of upper ; 



palatal folds in one series leiicochila. 



. Three transverse parietal plates feddeni. 



