LA Sey te 
THE NATURAL HISTORY SURVEY. Nay 
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 
Brien22: 
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 
E1G; 233 
Radula of PoLyYGyRA 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). 
