COMPARISON OF APPENDAGES. 67 



of some of the phyllopods. The shaft is a broad unsegmented blade. The setse are slender, 

 delicate, flattened, and a little longer than the width of the shaft. The exopodites of 

 this genus point forward all along the body. In Kootenia the exopodites are like those 

 of Ncolcnus, but with a narrower shaft. The exopodites of Ptychoparia appear to be very 

 much like those of Triarthrus, but the shaft is probably not segmented. 



The endopodites of the thorax of Triarthrus, Cryptolithus, and Acidaspis show pro- 

 gressive modification from front to back in the broadening of the individual segments and 

 the assumption by them of a triangular form. Not only do the individual segments become 

 more triangular from front to back, but more of the segments of each endopodite become tri- 

 angular. This modification has so far been seen in these three genera only. The individ- 

 ual segments, except the distal ones, seem to be flattened in all these genera. The distal 

 end of the terminal segment of each endopodite of Triarthrus bears three small movable 

 spines, and each of the segments usually bears three or more spines, located in sockets along 

 the dorsal surface and at the anterior distal angle of each segment. The endopodite of 

 Cryptolithus is bent backward at the carpopodite and this segment is always thickened. At 

 the distal end of the dactylopodite there is a tuft of spines, the triangular segments have 

 tufts of spines on their posterior corners, and there are groups of spines also in the neigh- 

 borhood of the articulations. 



The endopodites of Ceraurus, Calymcne, and Isotclus are all relatively slender, the seg- 

 ments are parallel-sided, and there seems to be no particular modification from front to back 

 of the thorax. The endopodites of Isotelus are short, the entire six segments of one being 

 but little longer than the coxopodite of the same appendage. The segments of the endopo- 

 dites of Neolenus are mostly short and wide, and at the distal end of the terminal segment 

 there are three stout spines. In Kootenia the endopodites are long and very slender. The 

 endopodites of Ptychoparia are too poorly preserved to show details, and those of the thorax 

 of Acidaspis likewise reveal little structure, but they seem to have the triangular modifica- 

 tion, and to turn back somewhat sharply at about the position of the carpopodite. 



Pygidium. 



Beecher showed that in Triarthrus there was a pair of appendages on the pygidium for 

 every segment of which it is composed except the last or anal segment (protopygidium). 

 Walcott has since shown that in Neolenus this segment bears a pair of cerci, and Beecher's 

 drawings show that in his later studies he recognized a spinous plate, the possible bearer 

 of cerci, on the anal segment of Triarthrus. The appendages of the anal segment have not 

 yet been seen on other species of trilobites. 



The appendages of the pygidium do not show any special modifications, but seem in 

 all cases to be similar to those of the posterior part of the thorax. In Cryptolithus all the 

 pygidial appendages are short and remain beneath the cover of the dorsal test, while in 

 Triarthrus and Neolenus they extend behind it. 



In the latter genus the endopodites of the pygidial appendages appear to be practi- 

 cally identical in form with those of the thorax, the individual segments being perhaps a 

 little more nearly square in outline. Like those of the thorax, the segments of the pygidial 

 endopodites bear numerous short spines. The caudal cerci are richly segmented, slightly 

 flexible, spinous tactile organs. They are symmetrically placed, nearly straight when in their 

 natural position, and make an angle of about 75 with one another. They appear to be 



