206 



PROCEEDINGS OF THE ACADEMY OP 



H. hirsuta (pi. IX., fig. 6) has 22—1—22 teeth ; 10 perfect 

 laterals. 



H.germana (pi. IX., fig. 5) has 28—1—28 teeth; 12 perfect 

 laterals. Fig. 6 shows one of the few marginals which have the 

 outer cusp bifid. 



H. monodon (pi. IX., fig. 4) has 21 — 1 — 21 teeth ; 10 perfect 

 laterals. Morse gives 28 — 1 — 28 teeth. 



H, labrosa, maxillata, and Edgariana not examined by me. 



Fig. 49. 



Jaw of Helix 

 appressa. 



Subgenus Triodopsis. 



Jaw stout, arcuate, low, wide, ends but little attenuated, blunt; 

 cutting margin without median pi'ojection ; anterior surface with 

 numerous decided, separated ribs, denticulating either 

 margin. There are about 15 in palliata ; 10 in ob- 

 atricta ; 15 in appressa; 14 in injiecta ; 10 in Ru- 

 geli ; 14: in fallax; oxer 10 in Ho2:)elo7iensis ; over 12 

 in Harfordiana ; 1 1 in loricata ;^ o\by \0 in triden- 

 tata. I have not examined H. Mullani^ and vultuosU' 

 The subgenus is almost exclusively North American. Two 

 Central American species have, however, been described, and one 

 European species, H. personata, Lam. This last is said by Mo- 

 quin-Tandon to have 3 — 5 separated ribs upon its jaw, while our 

 American species, as shown above, have numerous ribs. 



Triodopsis does not differ from Mesodon or Polygyra in the 

 character of its jaw. Stenofrema, on the other hand, is readil}' 

 distinguished by having the ribs broader and more crowded ou 

 its jaw- 

 Fig. 50. 



Lingual dentition of Helix appressa. 



The general arrangement of the teeth on the lingual membrane 

 is shown in fig. 50. The characters of the individual teeth are 

 given on pi X. I have selected H. appressa (fig. 7) to show these 



' The ribs are more crowded in this species. 

 2 Probably identical with devia. 



