June, iQ^i.l CraMPTON" : EvOI.UTIOX OF THE AXTHROPODA. 85 



mandibles of certain insects in havincr no palpus, or molar rej:^ion 

 differentiated from the incisor region. On the other hand, the 

 mandible of Caprclla resembles that of certain insects in having a 

 short molar protuberance " mo," and a shorter and stouter type of 

 mandible than is present in many Crustacea. The tendency toward a 

 shortening of the mandible is also exhibited by Ligia (Fig. 36) and 

 other isopods, and the molar process " mo " becomes blunter and 

 stouter in Ligia. Through a further shortening and through the 

 reduction of the basignath, or basal region of the mandible shown in 

 Fig. 36, the type of mandible found in certain pterygotan insects 

 might easily be derived, and the question naturally arises as to 

 whether the small sclerite called the basimandibula (or the " tro- 

 chantin " of the mandible) which is situated at the base of the 

 mandible in certain orthopteroid insects, may not correspond to the 

 demarkcd basal region of a mandible such as that shown in Fig. 36, 

 in which there is a slight indication of a division of the mandible into 

 a basal and a distal portion by the transverse dotted line shown in the 

 figure. While I would not deny the possibility of such an explanation 

 of the basimandibula (or mandiliular '" trochantin ") in insects, I am 

 more inclined to regard the formation of tliis l)asal sclerite or basi- 

 mandibula as the result of a chitinization of the articulatory mem- 

 brane at the base of the mandible in insects. 



Relationships Ixdic.\ted by ^Maxdihular Appendage. 



As was stated at the beginning of this paper, the mandibles alone 

 can furnish l)ut a small portion of the evidence of relationship, which 

 must be drawn from as many sources as possible, and should include 

 not only the study of anatomical details, but also that of the embryo- 

 logical development of the forms in question, as well as their habits, 

 immature stages, and all otlier features having a direct bearing upon 

 the subject. Taken in connection with these other features, however, 

 the nature of the mandibular appendage in the various arthropodan 

 groups is of considerable value in determining the lines of develop- 

 ment and the interrelationships of these groups, and it may be of 

 interest to point out some of the indications of relationship between 

 certain of the groups furnished by an examination of the mandibles. 



The character of the mandibular limb (or any of the mouthpart 



