No. 549] A CASE OF POLYMORPHISM 539 



these difficult structures by systematists who ran devote 

 but little time to a given point. I find the two rami are 

 never alike at the very tips. I am not speaking now of 

 the delicate lamellate teeth already mentioned which are 

 a little distance removed from the tips, but of the very 

 extremities. Of these latter, one is hi (id, or ends in two 

 delicate tips; even these again are never quite sym 

 metrical, but the one which is toward the animal or 

 posterior is a little smaller and shorter, furthermore, 

 the split in the tip of this jaw is not a simple cleft such 

 as one might produce by splitting the end of a stick with 

 a knife, but is a triangular groove, the base or open side 

 of which is toward the inside or concave aspect of the 

 ramus, the apex toward the outside. As aforesaid, this 

 cleft divides the tip of the ramus, but it is also continued 

 on the inner aspect of it considerably farther than it 

 extends on the outer, becoming thus shallower and 

 shallower as it extends farther from the divided tip. 

 The opposite ramus is not bifid, but tapers to a point, and 

 the tapering is of such a nature that the jaw near the 

 tip is more or less triangular in cross section, so as to fit, 

 not only into the cleft between the tips of its fellow 

 ramus, but farther into the triangular groove on its inner 

 side as well. Thus these delicate chitinous jaws, when 

 closed, lock together in double manner. 



The study of hundreds of examples of the trophi of 

 the humped rotifer as it occurred in the material first 

 examined left upon the mind of the writer a very distinct 

 impression of the minute delicacy of detail and very great 

 uniformity which prevails in these structures. Varia- 

 bility seemed almost wholly confined to the matter of 



Turning briefly to the trophi of the campanulate type, 

 I will say that they differ regularly from those just 

 described, not only in the features shown in the figure, 

 such as general size, breadth of rami, more acute angle 

 of the lower inner tooth, etc., but in other marked features 

 besides. The inner tooth, smaller in proportion as well 



