PYRAMIDULA. 51 



last obstructed far within by several pairs of elevated lameflce upon 

 the basal-outer wall (fig. 32). Lip thin, simple. Type H. semi- 

 plicata Pfr. pi. 14, fig. 32, 33. 



Anatomy unknown. Distribution, Madeira. This group is prob- 

 ably a modification of Goniodiscus. 



P. semiplicata Pfr. iii, 44. P. calathoides Paiv. iii, 44. 



gueriniana Lwe. 



Subgenus HELICODISCUS Morse, 1864. 



Helicodiscus MSB., Obs. Terrest. Pulm. Maine, p. 25, type H. 

 lineata Say. 



Shell small, disk or coin-shaped, with flat spire and broad, shallow 

 umbilicus. Whorls numerous, convex and closely coiled, spirally 

 striated or 1 irate, the last whorl having one or several pairs of tuber- 

 cular teeth within, situated upon the basal-outer wall. Aperture 

 lunate, lip thin, simple. Type P. lineata Say, pi. 14, figs. 29, 30, 31. 



The shell lies perfectly flat upon the posterior end of the foot, the 

 eye-peduncles standing nearly vertically ; posterior end of the long 

 and narrow foot conspicuously furrowed above, very short behind 

 the mantle (pi. 14, figs. 47, 48, P lineata). 



Jaw arcuate, striate, the striae diverging somewhat from the median 

 line; median projection inconspicuous (pi. 15, fig. 1, P. lineata). 



Morse's figure represents the jaw as less arcuate and pointed at 

 the ends. The jaw figured on my plate, however, seems to be per- 

 fect, although the ends are blunt. 



Radula having about 77 rows of 12'M2 or 13'M3 teeth. The 

 central tooth is decidedly narrower than the laterals, its mesocone 

 very short, side cusps minute. Laterals with large square basal- 

 plates, the mesocone as long as the basal-plate, entocone and ectocoue 

 equally developed, strong, with short cutting points. Marginals low, 

 wide, the ectocone bifid or trifid (pi. 11, fig. 24, P. lineata). 



These minute snails live upon decaying wood. The most con- 

 spicuous features of the dentition are the tricuspid lateral teeth, 

 recalling those of Stephanoda or Charopa, and unlike the teeth of 

 Pyramidula generally, in which the entocoues are as a rule absent. 

 The splitting of the ectocoues of the marginal teeth is correllated 

 with the small size of the creature, snails of many groups assuming 

 the Pupa-like form of marginal teeth when the size of the animal 

 becomes minute. 



