June, I9JI.1 CRAMrXON : EvcH.UTION OK TIIK AnTHROI'ODA. 71 



the limits ( uliich luive un jointed gnathobases in tlie two .cfroups). 

 and by the character of the segments and pygidium in trilobites and 

 immature merostomes such as Liwuliis. On the other hand, the 

 trilobites are very Crustacea-like in having biramous limbs, and other 

 features strongly suggestive of crustacean affinities, and I would not 

 minimize the fact that the trilobites are clearly annectant between the 

 Crustacea and Merostomata ; but, as I have pointed out in an article 

 in the American Xaturalist, and in the 51st Rei)ort of tlie Entomo- 

 logical Society of Ontario ( Crampton, 1919A and mjkjB) the ma'n 

 trend of the trilobitan developmental tendencies seems to lead more 

 toward the development of the merostome typo of arthropods, than 

 toward the main line of the Crustacea and their descendants the 

 myriopods and insects. 



Raymond, 1920, wlio regards tlie trilobites as the ancestors of 

 other arthropods, does not differ from me in this matter as funda- 

 mentally as his paper in the American Xaturalist would imply, since 

 I too regard the trilobites as very close to the ancestors of arthropods 

 in general (as was stated in the article in the Report of the Entomo- 

 logical Society of Ontario for 1919) ; but I do not consider the 

 trilobites as actual ancestors of the Crustacea such as .If^iis, etc. (and 

 consequently of higher Crustacea also), since the .\podidc'e were 

 contemporaneous with certain trilobites, and the earlier trilobites 

 combine in themselves so many apodid and crustacean features, that 

 I cannot avoid the conclusion that the first arthropods were more of 

 the nature of trilobitan-Crustacea (or crustacean-Trilobita) rather 

 thai> pure trilobites; and the inherent tendencies which flowed into 

 the purely trilobitan side of the early artliropodan lines of develop- 

 ment are mostly those which lead to the merostome type, rather than 

 to the types of development exhibited by the Crustacea, and their 

 descendants the myriopods and insects. 



To return to the subject of the origin of the mandibles of Crus- 

 tacea and their allies, it would appear that the precursors of the 

 mandibles were leg-like appendages of the biramous type (see text- 

 figure I. and Fig. 9 of Plate \'I ) in which the basal segment became 

 modified for holding or comminuting the food, while the endopodite 

 ("en" of textfigure i, and Fig. 9) or inner branch of the limb, still 

 served to aid in the locomotion of the creature, and the exopodite 



