EASTERN PROVINCE INTERIOR REGION SPECIES. 283 



TRIODOPSIS, Raf. 



Animal heliciform, mantle posterior, other characters as in Patula. 



Shell imperforate or nmbilicated, orbicularly depressed or subglobose, 

 more or less obliquely striated; fig 301. 



whorls 5-7, the last somewhat de- 

 flexed in front; aperture sinuously 

 coarctate, subtriangular ; jteristome 

 white, thickened, broadly and an- 

 gularly reflexed, usually dentate ; Animal of t. paiuata. 

 parietal wall of the aperture with a strong, obliquely entering denticle. 



The subgenus inhabits almost exclusively ISTorth America, especially 

 the Eastern Province. Two Central American species have, however, 

 been described, and one European species, personata, Lam. This last is 

 said by Moquin-Tandon to have 3-5 separated ribs upon its jaw, while 

 our American sj)ecies, as shown below, have numerous ribs. 



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

 ting margin without median projection; anterior surface 

 with numerous decided, separated ribs, denticulating either 

 margin. There are about 15 in palUata ; 10 in obstricta ; 15 

 in appressa ; 14 in inflecfa ; 10 in Rugeli ; 14: in f alia x ; over 

 10 in Eopetonensis ; 17 in Van Nostrandi ; 14: in introferens ; 

 over 12 in vultuosa ; 11 in loricata ; * over 10 in tridentata. 



Triodopsis does not differ from Mesodon or Polygyra in the character 

 of its jaw. Stenotrema, on the other hand, is readily distinguished by 

 having the ribs broader and more crowded on its jaw. 



The general arrangement of the teeth on the lingual membrane is as 

 in Patula. The characters of the individual teeth are given on Plate 

 VII of Terr. Moll., V. I have selected appressa (Plate VII, Fig. Q) to 

 show these characters, comparing the dentition of the other species 

 with it. The centrals are longer than wide ; the base of attachment has 

 its outer, lower, lateral expansion but little developed, its lower margiu 

 incurved, its upper margin squarely reflected ; the reflection is stout, 

 with subobsolete side cusps but well-developed side cutting points, and 

 a stout, short median cusp, bearing a cutting i)oint which does not 

 reach the lower margin of the base of attachment. The laterals are 

 like the centrals, but, as usual, asymmetrical by the suppression of the 



* The ribs are more crowded in this species. 



