June, 1921.] CrAMPTOX : EvOLUTIOX OF THE AnTIIROPODA. 79 



surface (or the gnathobase) of the basal segment of the mandibular 

 appendage. On this account Wood-Mason's statement that the molar 

 process " / "' of Macliilis (textfigure 6) represents the endopodite 

 "en" of textfigures 5, 4. 3. 2, and i. is most astounding, and it is 

 very difficult to understand how anyone who has studied the Crus- 

 tacea at all. could come to such an unwarranted conclusion. 



IXTERPRETATIOX OF pARTS OF AX Ix.^ECT's MaXDIBLE. 



From the foregoing discussion, it should be evident that an in- 

 sect's mandible rejiresents only one segment of the original limb 

 forming the niandil)ular appendage of trilobites. Crustacea, etc.. and 

 neither the exopodite " ex" of textfigure i, nor the endopodite "en " 

 of te.xtfigures i, 2. 3, 4. and 5. are represented in the mandible of an 

 insect. Furthermore, the division of MachUis' mandible into a basal 

 region or basignath. and a distal region or distignath, by the suture 

 from " b " to " c " in textfigure 6. is a purely secondary one, and in 

 fact is foreshadowed in the mandibles of certain Crustacea such as 

 our common southern shrimp (and a hint of this division is shown in 

 the mandible of the crustacean Ligia — see Fig. 36. Plate VIII). 



The evolution of the mandibles shown in textfigures i. 2. 3. 4. and 

 5, is in complete agreement witli the relative positions assigned the 

 arthropods in question from their general anatomical features, and I 

 do not see how anyone can deny that the series in all probability 

 represents the stages through which a specialized mandible such as 

 that of Apscndcs (textfigure 5) has passed in assuming its present 

 condition. We are therefore justified in assuming that the mandible 

 proper {i.e., exclusive of the palpus) of Crustacea is composed of one 

 and only one segment of the original mandibular appendage; and if 

 we compare the tentorio-basignathal muscles attached to the basal 

 region (basignath) of Macliilis' mandible (Fig. 5. Plate VI) with the 

 muscle attached to the base of the mandible of the crustacean shown 

 in Fig. 2 (Plate VI) it is evident that the muscles are practically the 

 same — consequently the regions to which they are attached are homo- 

 logous, and the apparent basal segment of Macliilis. mandible (Fig. 

 5) is therefore merely the basal region of the mandible of the crus- 

 tacean shown in Fig. 2. ivhicli is composed of only one segment. It 

 follows from this, that the subdivision of the mandible of MachUis is 



