Tune. i9-:i.l CrAMPTI^N : EvOLVTIOX OF THE A NTH ROPODA. 87 



The gnathobases " <jb" of the merostomes shown in Figs. 12 and 

 16 are clearly homologous with the gnathohases " gb " of the trilo- 

 bites shown in Figs. 18 and 9, since in each instance the gnathobase 

 IS formed by a mesal i)roIongation of the entire basal segment of the 

 limb. On the other hand, the so-called gnathohases " gb " of certain 

 Crustacea such as those shown in Figs. 13 and 21, appear to be 

 slightly ditYerent outgrowths or appendages of the segment; and in the 

 crustacean shown in Fig. 20, the endites " ci " of the several segments 

 of the limb, which are homologous with the gnatholiase or endite 

 lal)eled " gb " in the basal segment, have taken on the appearance of 

 segmented appendages of the segments rather than mesal prolonga- 

 tions of the entire segment — although it must be admitted that the 

 endite " gb " of the basal segment of the limb shown in Fig. 20. ap- 

 proaches more closely to the trilobitan type of gnathobase (" gb" of 

 Fig. 18). The greater similarity between the gnathohases of trilo- 

 bites and merostomes (i.e., " gb " of Figs. 18. 15. and 12) than be- 

 tween the gnathobase of trilobites and the endites of Crustacea in 

 general (compare "gb" of Fig. 18 with "gb" of Figs. 21 and 13. 

 or "ci" of Figs. 19 and 20). however, would tend to throw the 

 trilobites slightly nearer the merostomes than the Crustacea, when 

 taken into consideration with certain other features of resemblance 

 in the groups Trilobita and Merostomata. 



As was remarked above, the mandibles of the Branchiopoda (Figs. 

 22 and 27,) are not very much like the mandibular appendage of 

 trilobites (Fig. 9) despite the close relationship between the two 

 groups; but the mandibles of the Copepoda and Ostracoda (Figs. 30 

 and T,i) have still retained both the exopodite "ex" and the endo- 

 podite "en" of a typical biramous limb (Fig. 9, "ex" and "en"), 

 a]id it would appear that the masticatory portion of the mandible of 

 the Crustacea shown in Figs. 30 and ^^ represent true gnathohases 

 " gb" of a trilobitan limb (Fig. 9) since the structures in question 

 are mesal prolongations of the entire basal segment in both instances. 

 I would therefore maintain that the Copepoda (and the Ostracoda 

 also) are quite closely related to the Trilobita. although the former 

 belong to the subphylum " Eugnathata " in which the mandibles have 

 passed beyond the walking-leg stage and have become true jaws; 

 while the trilobites appear to be somewhat more closely associated 

 with the subphylum " Podognathata " in which the appendage homo- 



