130 HEFFERAN. [ VOL. II. 



I. Method. 



The specific character selected for investigation was the 

 number of teeth on the jaw. This number is commonly stated 

 by authors in descriptions of species. 



The jaws are two in number, from I to 3 mm. in length, 

 brown, horny, curved, and serrated along the inner or falcate 

 margin. They are at the extremity of a large exsertile pro- 

 boscis which is usually retracted in alcoholic specimens, so 

 that in order to free the jaws it is necessary to cut down the 

 median line of the head, ventrally, through to the inside of the 

 muscular proboscis. By turning the head backwards the jaws 

 can be made to extrude, and the teeth counted by means of a 

 hand lens. In the specimens killed later in the season by a 

 slowly killing fluid, the jaws remained extruded. 



In counting, some difficulty was experienced in fixing a limit 

 in either direction, at the curved, distal end in those cases in 

 which very fine teeth ran to the tip, and particularly at the 

 proximal extremity where the outlines of the teeth were indefi- 

 nite. In order to count these it was necessary to pull out the 

 jaws gently with a forceps, and to free the bases from connect- 

 ive tissue, carefully, without entirely separating the jaws from 

 the head. Since the line of division between the free, promi- 

 nent teeth and the undeveloped ones, buried in a chitinous 

 band, was always distinct, the method was adopted of counting 

 them separately, and of including in each set all that showed 

 well-formed outlines when held up against a strong light and 

 viewed through a lens from the dorsal side. Those connected 

 by the chitinous band were called indefinite teeth, the rest 

 the definite teeth. Totals were found by adding. 



2. Results. 



I. TABLE OF DISTRIBUTION OF FREQUENCIES. 



9 10 ii 12 13 14 



95 116 100 39 7 



27 IO2 114 89 46 10 



