June, 1921.] Crami'Tox: Evoll'tiox of the Axthropoua. 73 



ment of the endopodite is tlirown forward by tlie lengthening of tnc 

 area from " a " to " c " in the merostome, as may be seen by com- 

 parinj,'- the extent of the area from " a " to " c " (marked with a black 

 margin id make the comparison easier) in textfigures i and 2. In 

 Fig. 14 (Plate \'II) of the gnathobase of a limb of Liniulus, the 

 proximal spine-like projections are closer together, and stouter than 

 the distal ones, and on this account, I have represented the spine- 

 like i)rojections in the area labeled " / '" in the merostome shown in 

 textfigure 2, as though they were becoming more massed together than 

 is the case with the spines in the stippled area " c " of tne gnathobase. 



The chief differences between the stages shown in textfigures I 

 and 2 are thus seen to be the loss of the exopodite ■' ex," the shifting 

 forward of the endopodite " en " through the lengthening of the 

 area from ''a" to " c '' (margined in black), the shortening of the 

 area from " d" to " c " (i.e., the stippled area), and the differentia- 

 tion of the " masticating " area into an upper area " e " and a lower 

 area "/"' in which the spine-like projections become more "massed 

 together." While the merostomes do not stand in the direct line of 

 descent of the Crustacea, but are on a side line leading to the arach- 

 noid type of arthropod, the merostomes have nevertheless retained 

 the condition of the limbs in a fairly primitive state, approaching the 

 condition which was doubtless characteristic of the limbs of a number 

 of primitive forms a little more modified tiian the trilobites; and on 

 this account the condition exhibited by the merostome appendage 

 may be taken as representative of the stage of development inter- 

 mediate between the trilobite shown in textfigure i and the crus- 

 tacean shown in textfigure 3, despite the fact that the character of 

 the body as a whole, in the merostomes, exhibits more arachnoid 

 than crustacean features. 



In the crustacean Xehalia, whose mandible is shown in textfigure 

 3, the mandibular appendage has taken on the character of a true 

 mandible, through the reduction of the endopodite " en " to a three- 

 segmented palpus, the dift'erentiation of the "•masticatory" area into 

 a well defined incisor or biting area " c' " (stippled in the figure), and 

 a molar or grinding process " /," which is probably formed by the 

 crowding together of the spine-like projections of the area labeled 

 "/."' and the elongation of this area to form the molar process "/" 



