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THE AMERICAN NATURALIST [ Vol. XLVI 



bending over of the corner of a ridge by the pressure of a 

 cover glass seemed improbable, and I immediately put it 

 to test in various ways. For one, I extracted a large 

 number of the trophi by means of potassic hydrate in 

 deep watch-glasses. Here there was obviously no pres- 

 sure, yet the structures in question were quite visible, 

 even before the trophi had been transferred to a slide, or 

 touched by any instrument. Moreover, these thin 

 lamellate teeth are never quite symmetrical on the two 

 rami, and this delicate discrepancy is always on the same 

 side of the animal, as I ascertained later in stained and 

 mounted preparations. I carried my study of the trophi 

 farther by mounting many which I had extracted in deep 

 hollow-ground slides, including with them a small bubble 

 of air. By giving the slide a quick tilt the air bubble 

 could be made to strike and overturn the trophi in dif- 

 ferent ways. By then replacing the slide quickly under 

 the microscope, views could be had of the trophi before 

 they had settled to the ordinary horizontal position. A 

 half hour of such attempts readily furnished views of 

 every part of the trophi, seen from almost every possible 

 angle. Portions so thin as to be invisible in one view 

 become visible in another; optical sections at all points 

 make possible the arrival at the correct form. I regret 

 that in my drawing I have been able to show so little of 

 the delicate complexity of these structures; but Bous- 

 selet's view of their structure— that "the chitinous ma 

 terial is bent at right angles throughout the length of 

 the rami, forming an inverted L in cross section" is 

 certainly very far from correct. The structure varies at 

 different points ; ridges thicken and fade out in complex 

 and sinuous fashion, quite as we should find them in the 

 complex chitinous jaw of an insect, or, for that matter, 

 in the jaw-bone of a mammal. 



The tips of the jaws are interesting, and I find no 

 description of the jaws of any species of Asplanclina, by 

 any author, which coincides with my observations. 

 Nevertheless, this may be due to the inadequate study of 



