300 C. E. Beecher — Pygidium of Triarthrus. 



is composed of segments which are considerably expanded 

 transversely, thus making a paddle-like organ, the anterior edge 

 of which is straight, while the posterior one is serrated by 

 the projecting points of the expanded segments. These points 

 bear small bundles of setae (Plate VII, figure 2). 



The extreme posterior endopodites are made up of endites 

 having a width two or three times greater than the length. A 

 young individual (Plate VII, figure 3) presents pygidial endop- 

 odites, having this character while the lower thoracic seg- 

 ments (not represented in the figure) bear endopodites similar 

 in form to a YI , in figure 2. As these phyllopodiform append- 

 ages are the youngest and most immature on an adult Triar- 

 thrus, and are the prevailing form on the pygidium of an 

 adolescent specimen, they probably indicate some primitive 

 feature or simple type of trilobite leg. Their resemblance to 

 the endopodites of Apus, figure 4, or to a typical phyllopod 

 limb is very marked. 



There is little apparent change in the structure of the exop- 

 odites from the anterior thoracic segments to the pygidium, 

 as throughout this length they retain the large basal joint with 

 a slender many jointed end carrying a long fringe of closely 

 arranged setse. As shown in figure 1, these fimbriae overlap, 

 and project behind the pygidium, forming with the endopo- 

 dites a fin-like expansion on each side, especially well adapted 

 to propel and guide the animal in swimming. They may have 

 served also as egfs; carriers. As far as known, there was but a 

 single pair of biramous limbs to each annulation, or potential 

 segment, of the pygidium. 

 Yale Museum, New Haven, Conn., March 9th, 1894. 



EXPLANATION OF PLATE. 



Triarthrus Becki Green. 



Figure 1. — Dorsal side of pygidium, showing fringes of exopodites (b) and ends 

 of endopodites (a). The last free thoracic segment is represented 

 in outline. Taken from an entire individual, x 1 6. 



Figure 2.— Ventral view of pygidium with appendages well preserved on one 

 side, showing expanded setiferous endites of endopodial limbs 

 (a 1 , a T , etc ) and exopodites (b). TJie last free thoracic segment is 

 represented in outline. Taken from an entire individual, x 16. 



Figure 3. — The endopodites (somewhat displaced) from right side of pygidium 

 of small specimen. The two end segments of upper limb are 

 restored. The lower limbs are minute and rudimentary, x 60. 

 Utica Slate. Near Rome, N. Y. 



Figure 4. — Endopodites from right side of fourth larval stage of Apus, enlarged. 

 (After Claus) 



