MORPHOLOGY OF TRIARTHRUS 215 



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 forward 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, espe- 

 cially 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 backward, 

 while those on the head are directed more or less forward. 

 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, correspond- 

 ing to the mandibles of higher Crustacea, has not become 

 clearly differentiated from the rest of the series, and appar- 

 ently has not lost the exo- and endopodial branches. 



Few changes of importance can be traced in the exopo- 

 dites, 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 

 navigation. The form of the animal and the multiphcity of 

 locomotor organs were well adapted for rapid motion either 

 along the sea-bottom or through the water. 



The youngest and most immature limbs are on the pygid- 

 ium, and in a young trilobite they are very much like those 

 in the larval ^7>MS*and are typically phyllopodiform. Ac- 

 cording to the law of morphogenesis, these limbs may be 



