252 C. E. Beecher — Morphology of Triarthrus. 



The dorsal view represented on Plate YIII 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 seg- 

 ment, 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 representing the best 

 portions of three individuals. 



The ventral view, Plate YIII, is based mainly upon two 

 very excellent specimens. One was figured on Plate IY, vol. 

 xv, of the American Geologist, and another, since found, 

 nearly completes the ventral aspect. The under side of the 

 head and pygidium was carefully compared with all the avail- 

 able material, and no attempt was made to supply any charac- 

 ters except as to the exact number of joints in the endopodial 

 cephalic elements and the precise form of the cephalic exopo- 

 dites, 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." 

 The entire length of a thoracic leg, including the coxal joint, 

 is nearly equal to the width of the body at that point, and 

 about half the length projects beyond the pleura. 



The limbs of the head diminish in length forwards until the 

 anterior pair scarcely extends beyond the border of the cephalon. 

 The anterior thoracic legs are the longest, and there is a grad- 

 ual shortening backward in the series, especially noticeable after 

 passing the fifth, those at the extremity of the pygidium being 

 about one-ninth the length of the first thoracic leg. Their 

 position is also of interest. At the posterior extremity they 

 point almost directly backwards, while those on the head are 

 directed more or less forwards. Between these two extremes, 

 all the intermediate positions occur in regular order. 



* Structure and Appendages of Trinucleus, C. E. Beecher. This Journal, vol. 

 xlix, April, 1895. 



