THE NATURAL HISTORY SURVEY. 



157 



two when the animal is extended One specimen gave eighty- 

 eight beats in a minute. 



Jaw: Very long and narrow, slightly arcuate, "the ends 

 knob-shaped; the anterior and posterior surfaces are armed 



FIG. 22. 

 Jaw of POLYGYRA THYROIDES Say. (Original.) 



with thirteen stout ribs which denticulate both dorsal and ven- 

 tral margins (Fig. 22). 



Radula: With 58-1-58 teeth; central tooth with a long and 

 narrow base of attachment, laterally expanded at the lower 

 outer corners; reflection as long as base, unicuspid, the cusp 

 being long and narrow; lateral teeth similar to centrals but 

 asymmetrical (eleven perfect laterals); marginal teeth at first 

 similar to laterals, but soon the cusp elongates, an outer side 



FIG. 23. 



Radula of POLYGYRA THYROIDES Say. (Original.) c, central tooth; 

 1, first lateral; 13, second marginal; 27, sixteenth marginal; 42, 45, '.thirty- 

 first and thirty-fourth marginals, 49, thirty-eighth marginal. 



cusp is developed, the larger cusp becomes bifid, and toward 

 the end of the membrane the marginals become short and wide 

 and two outer side cusps appear. All of the cusps are pro- 

 vided with very long and stout cutting points'(Fig. 23). 



