214 STUDIES IN EVOLUTION 



great assistance in forming a conception of general trilobite 

 morphology. 



The dorsal view represented on Plate IX is from a camera 

 drawing based upon three specimens of about the same size. 

 One gives the entire series of legs down to the ninth free 

 segment, with the exception of the exopodites of the head, 

 which are supplied from a second individual. In the third 

 specimen the anterior appendages are bent and irregularly 

 arranged, while from the ninth backward to the end of the 

 pygidium they are complete and uniformly extended. The 

 figure is, therefore, a restoration only in so far as represent- 

 ing the best portions of three individuals. 



The ventral view (Plate IX) is based mainly upon two 

 very excellent specimens. One was figured on Plate IV, 

 vol. XV, of the American G-eologist, and another, since found, 

 nearly completes the ventral aspect. The under side of the 

 head and pygidium was carefully compared with all the 

 available material, and no attempt was made to supply any 

 characters except as to the exact number of joints in the 

 endopodial cephalic elements and the precise form of the 

 cephalic exopodites, which, from every character observed, 

 and from analogy with similar structures elsewhere, were as 

 represented. 



So many specimens preserve the appendages in the position 

 shown in the figures, that this must be recognized as natural, 

 and one likely to have been assumed by the living animal 

 when extended. Few, however, show the details of the 

 limbs with sufficient clearness to enable one to make out all 

 their joints and more minute characters. 



In comparison with what is now known of the appendages 

 of several other genera of trilobites, especially Trinucleus,* 

 those of Triarthrus seem to have been exceptionally long. 

 On this point Bernard, in a letter to the writer, suggests 

 that " Triarthrus must have been a sort of ' Daddy longlegs ' 

 among the Trilobites, as Scutigera is among the Myriapoda." 



* Structure and Appendages of Trinucleus. Amer. Jour. Sci. (3), XLIX, 

 April, 1895. 



