194 C. E. Beecher—The Morphology of Triarthrus. 



The ventral view, Plate IX. is baser! mainly upon two very 

 excellent specimens. One was figured on plate iv, vol. xv, of the 

 American Geologist, and another, since found, nearly completes the 

 ventral aspect. The under side of the head and pygidium was care- 

 fully 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, 1 those of 

 Triarthrus seem to have been exceptionally long. On this point 

 Bernard, in a letter to the writer, suggests that " Triartlims 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 forwai-ds until the 

 anterior pair scarcely extends beyond the border of the cephalon. 

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

 shortening backward in the series, especially noticeable after passing 

 the fifth, those at the extremity of the pj'gidium 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. 



The gnathobases, or coxopodites, become more and more 

 specialized anteriorly, growing broader and having their inner edge 

 denticulate, until on the head they function as true manducatory 

 organs. The second pair, however, corresponding to the mandibles 

 of higher Crustacea, has not become clearly differentiated from the 

 rest of the series, and apparently has not lost the exo- and endo- 

 podial branches. 



Few changes of importance can be traced in the exopodites, 

 though the latter are considerably reduced in size on the cephalon. 

 Over the anterior half of the thorax, they functioned as vigorous 

 paddles ; and on the pygidium their length and compact arrangement 

 made them overlap each other, thus producing two broad flaps, or 

 fin-like organs. The conclusion cannot be avoided that Triarthrus 

 must have been an active creature, and with its rows of endopodites 

 and exopodites it was as fully equipped as the bireme in classic 



1 "Structure and Appendages of Trinucleus, 1 '' C. E. Beeeher : Anier. Journ. 

 Science, vol. xlix, April 1895. 



