76 Journal New York Extomological Society, t'^'o'- xxix, 



of textfigure 4 (or Fig. 7) to execute, since the joints of the endo- 

 podite "en" of Mysis (textfigure 4) are of such a nature that the 

 endopodite is directed mesahvard instead of outward as in Nchalia 

 (textfigure 3). 



The black margined area from "a" to " c" is of proportionately 

 greater extent in textfigure 4 than in textfigure 3, and arches over 

 more pronouncedly in the former (Mysis) than in the latter (A''^- 

 balia) as may be more readily seen by comparing Fig. 7 with Fig. 

 10 (Plate VI). The molar region (" f " of textfigures 4 and 3, or 

 "mo" of Figs. 7 and 10) is essentially the same in both Crustacea, 

 but in the incisor region of Mysis (textfigure 4, or Fig. 7) the fol- 

 lowing parts become differentiated. The distalmost portion or apex 

 of the mandible proper retains its character of an incisor region 

 (labeled " c" in textfigure 4, or "in" in Fig. 7), but the processes 

 below it (i.e., " g" of textfigure 4, or " gf " of Fig. 7). called the 

 gnathofimbrium, may assume the character of a fringe of setae-like 

 structures, which are quite long and slender in the Crustacea shown 

 in Figs. 4, 2, etc., of Plate VL The function of this fringe is prob- 

 ably to sweep the food into the mouth cavity, or to prevent the 

 comminuted food from falling out of the mouth when the mandibles 

 are working. The so-called lacinia mobilis, labeled " li " in textfigure 

 4, or " hn" in Figs. 7, 4, etc., is probably formed by the fusion of 

 flattened seta-like structures like those forming the gnathofimbrium 

 below it. The articulatory region which bears the articulatory con- 

 dyle near the letter " a " in textfigure 4 bends outward more markedly 

 in Mysis (textfigure 4) than in Nchalia (textfigure 3) and this 

 tendency is quite pronounced in the following stages. 



In the stage of the evolution of the mandibles represented in 

 textfigure 5 (compare also Fig. 3, Plate VI) the profile of the black- 

 margined region from "a" to " c" is not quite so arched as in text- 

 figure 4, and this straightening out of the arch is apparently cor- 

 related with the "upward" growth of the incisor process {i.e., the 

 stippled area in textfigure 5) which becomes long and slender in 

 Apscudcs (textfigure 5, or Fig. 3). Diastylis, and other related forms. 

 The gnathofimbrium "g" of textfigure 5 (or " gf " of Fig. 3) is 

 much reduced in Apsciides (textfigure 5), and the lacinia mobilis " h" 

 of textfigure 5 (or "hn" of Fig. 3) is relatively small and unim- 



