TRIODOPSIS. 



301 



Fig. 196. 



Jaw of 

 T. appressa. 



TRIODOPSIS, Raf. 



Animal hclicifonn, nianllo posterior, other characters as in Pnfula, q. v. 

 Shell imperforate or umbilicated, orbicularly depressed or sub^lohose ; more 



or less obliquely striated ; whorls 

 ^'8- ^^^- 5-7, the last somewhat deflexed in 



front; aperture sinuously coarctate, 

 su])triangular ; peristome white, 

 thickened, broadly and angularly 

 reflexed, usually dentate ; parietal 

 wall of the aperture with a strong. 

 Animal of T. palliata. obliquely entering denticle. 



The subgenus inhabits almost 

 exclusively North 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 species, as shown above, 

 have numerous ribs. 



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

 cutting margin without median projection; anterior surface with 

 numerous decided, separated ribs, denticulating either margin. 

 There are about 15 in palliata; 10 in ohstricta; 15 in appressa; 

 J 4 in injiecta; 10 in Rugeli; 14 in fallax ; over 10 in Hopeto- 

 nensis ; 17 in Van Nostrandi; 14 in introferens ; over 12 in Harfordiana vultu- 

 osa ; 11 in loiicata;^ over 10 in (ridentata. I have not examined //. Mullani?' 

 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 arransement of the teeth on the lingual membrane is as in 

 Patula. The characters of the individual teeth are given on Plate VII. I have 

 selected appressa (PI. 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 margin 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 point 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 inner, lower, lateral ex- 

 pansion of the base of attachment and the inner side cusp with its cutting 

 point. The transition teeth are characterized by the gradual lesser propor- 

 tional development of the reflection, and greater development of the inner cut- 

 ting point; as the teeth pass outward, this point becomes bifid, the reflection 



1 The ribs are more crowded in this species. 



2 Probably identical wit devia. 



