Yol. 51.] 



POSITION OF THE TRILOBITES. 



353 



to function as jaws. The appendages of the 2nd segment, which 

 would be the nearest to the mouth in its new position, must still at 

 first have been behind the labium. 



In Apus we find a high degree of specialization ; the greatly 

 developed lateral lobes of the labium have grown backward so as 

 to overlap posteriorly the jaw-pieces of the third pair of appendages, 

 which are developed as enormous mandibles. 



In Idmulus we find a still further specialization. The labial 

 lobes have been forced back still farther, and appear as two small 

 appendages behind the jaw-pieces of the sixth pair of appendages. 

 Lastly, the large metastoma of the Eurypterids is probably a further 

 adaptation of the same labial lobes situated also behind the sixth 

 pair of appendages. 



We can thus trace through Triarthrus, Apus, and Limulus 

 A, B, C, m) the gradual 

 modification of the labium 

 from its primitive condition 

 as a ridge-like posterior 

 border of the mouth, 

 such as the ' bent annelid ' 

 theory demands, into the 

 pair of specialized and 

 enigmatical lobes such as 

 is found in the last- 

 named animal. We can 

 also trace equally clearly 

 through the same series the 

 gradual translocation of the 

 labium, or rather of its 

 lateral lobes, as they tra- 

 velled backwards from their 

 primitive position in front 

 of the second pair of ap- 

 pendages to their extreme 

 position behind the sixth 

 pair. 



The Mouth-formula. — The 

 original similarity of the 

 limbs of the head and of 

 the trunk, which the anne- 

 lidan theory demands, is 

 still found in Triarthrus. 



Passing by the first pair, 

 each of which was probably 

 from the first nothing more 

 than the cirrus of a notopo- 

 dium, as is frequently the 



case in the chaetopods, we find all the following appendages (ex- 

 cluding those of the pygidium) structurally alike. 



2 2 



