C. E. Beecher — Morphology of Triarthrus. 253 



The gnatliobases, or coxopodites, become more and more 

 specialized anteriorly, growing broader and having their inner 

 edge denticulate, until on the head they function as true man- 

 ducatory organs. The second pair, however, corresponding to 

 the mandibles of higher Crustacea, has not become clearly dif- 

 ferentiated from the rest of the series, and apparently has not 

 lost the exo- and endopodial branches. 



Few changes of importance can be traced in the exopodites, 

 though the latter are considerably reduced in size on the ceph- 

 alon. 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 f ally 

 equipped as the bireme in classic navigation. The form of the 

 •animal and the multiplicity 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 pygidium, 

 and in a young trilobite they are very much like those in the 

 larval Apus* and are typically phyllopodiform. According to 

 the law of morphogenesis, these limbs may be taken as of phylo- 

 genetic value and indicative of the primitive type of limb 

 structure. 



The whole series of endopodites anterior to the last two or 

 three show modifications from the phyllopodous type, the 

 change involving progressively from one to all of the endites. 

 The endopodites of the pygidium have a true phyllopodiform 

 structure, and are composed of broad leaf-like joints, wider 

 than long. This character is gradually lost in passing ante- 

 riorly, the distal endites being the ones first affected. By the 

 time the anterior pygidial limb is reached, the three distal joints 

 are longitudinally cylindrical. The ninth thoracic endopodite 

 shows a fourth endite becoming cylindrical, and on the first 

 and second thoracic legs even the proximal ones are thus modi- 

 fied, making all the endites of these limbs slender in form. 



This gradual modification of a phjllopodiform swimming 

 member into a long, jointed, cylindrical, crawling leg deserves 

 more than passing notice, for here, probably, better than in any 

 known recent form can the process and its significance be 

 studied. No living type of crustacean more nearly conforms 

 to the theoretical archetype of the class than do the trilobites, 

 and as Triarthrus belongs to an ancient Cambrian family, it 

 may be expected to retain very primitive characters. 



In this genus several causes evidently influenced the modifi- 

 cation of the appendages. First may be mentioned the speciali- 



