SECT. XIII THE TRILOBITES 229 



them to hold their own to-day, whereas Limulus and 

 Apus are probably the only surviving Crustacea which 

 retain the original position of the Annelidan antennae. 



As above pointed out, our interpretation of the head 

 limbs of the Trilobites rests largely upon our homo- 

 logising the large locomotory limbs with the sixth 

 pair of Annelidan parapodia, or with the first trunk 

 limbs of the Crustacea. Our adoption of this large 

 locomotory limb throughout all the primitive Crus- 

 tacea as the first trunk limb, for reasons given p. 44, 

 receives some support from Walcott's restoration, 

 where it lies behind the line which runs from side to 

 side, through the widest part of the head, which is 

 morphologically the line round which the body bent. 

 We have already seen, further, that Limulus — and we 

 shall see that the Eurypteridae, with some exceptions — 

 not only possessed the two pairs of antenna.^ but also 

 the large locomotory limbs as the sixth pair, i.e. 

 the first pair of trunk limbs. 



It should be mentioned that so far as these con- 

 clusions are based on the few sections published 

 in Mr. Walcott's paper, his conclusions are undoubt- 

 edly of much greater value than ours, inasmuch as 

 they were based upon a much more extended study 

 of sections, and of the whole Trilobite problem. We 

 have, however, to set, as against this, our claim to have 

 found in Apus a kc}' to the true understanding of 

 the morphology of all these primitive Crustacea. 



The form of the trunk limbs in the Trilobites does 

 not at first sight admit of any close comparison with 



