C. E. JBeecher — Pygidium of Triarthrus. 299 



scribed, after which it will be possible to review the present 

 knowledge of Triarthrus, together with its bearings on the 

 position and affinities of the Trilobita. 



Walcott first determined the presence of pygidial legs, and 

 in a restoration of the ventral surface of Calymene senaria, 

 one pair is represented to each annulation of the axis.* Mat- 

 thew (1. c.) figured and described an imperfect and poorly 

 preserved specimen of Triarthrus, which obscurely showed 

 two flaps or fin like extensions, beyond the lower edge of the 

 pygidium. These he considered as possibly anchylosed pygidial 

 limbs. 



The specimens here described leave little to be desired in 

 the way of perfect preservation. The lower side of a 

 number of individuals has been exposed, showing the ventral 

 membrane with the appendages attached. The dorsal shield 

 has also been removed from other specimens, and the append- 

 ages exposed from that side, so that a very complete idea may 

 be had of the character of the limbs, with many of the finer 

 details of structure. The small size of the pygidium of Tri- 

 arthrus, and the great number of elements in the appendages, 

 however, render it difficult to remove the shaly matrix com- 

 pletely. Moreover, the overlapping and crowded arrangement 

 of the limbs make the structure of the pygidial region seem 

 quite complicated. 



In Calymene, as worked out by Walcott, these appendages, 

 of which the endopodites alone are shown, are weak and slender, 

 and appear to show important differences from those of 

 Triarthrus. 



In Triarthrus, the thoracic legs near the head are composed 

 1 of comparatively long joints, the 



proximal ones only of both 

 endopodite and exopodite being 

 somewhat flattened and ex- 

 panded (figure 1). The func- 

 tions of these two branches are 

 clearly indicated, the endopodite 

 being adapted for crawling and the 

 exopodite for swimming. From 

 the anterior region of the thorax, 

 there is a gradual and progressive 

 change in both branches of the legs 

 increasing their functional powers 

 Figure 1. - Triarthrus Bechi ag sw j mm i™ organs. When the 

 Green; dorsal view of right legs . -,. . & ^ , . , 



of second and third free thoracic pygidium is reached, the enclopo- 

 segments. x 12. elites preserve their slender, jointed 



distal portion, but the proximal part 



* The Trilobite : new and old evidence relating to its organization. Bulletin 

 of the Museum of Comparative Zoology, vol. viii, No. 10, 1881. 



