SPECIMENS OF TRIARTHRUS. I S3 



from it, and the appendages of the left side from No. 206. Such of the endopodites as are 

 well preserved show from three to four segments projecting beyond the test, and the dacty- 

 lopodites have one or two terminal spines. The antennules are unusually well preserved and 

 have about forty segments each in front of the cephalon, or an average of five to one 

 millimeter. 



Specimens 209 and 210 are on a slab about 7X5.5 inches, and with them are twelve 

 other more or less well preserved individuals, all but one of which are smaller than these. 

 Two of the fourteen are ventral side up on the slab, which means dorsal side up in the rock. 

 Nine are oriented in one direction, two at exactly right angles to this, and three at an angle 

 of 45 with the others. If the majority of the specimens are considered to be headed north- 

 ward, then seven are so oriented, two northeast, one east, two south, one southwest, and 

 one west. 



Nine of the specimens show antennules. Five of these are specimens headed north, and 

 in all of them the antennules are in or very near the normal position. The antennules of 

 two, one headed east and the other west, are imperfectly preserved, but the parts remain- 

 ing diverge much more than do the antennules of those in the normal position. The indi- 

 vidual headed southwest has one antennule broken off, while the other is curved back so that 

 its tip is directed northward. Another one, headed south, has the antennules in the normal 

 position. These observations indicate that the specimens were oriented by currents of water, 

 rather than in life attitudes, and that the distal portions of the antennules were relatively 

 flexible. 



Measurements: The specimen (No. 210) is 20 mm. long, 9.5 mm. wide at the back of 

 the cephalon, and the antennules project 8 mm. in front of the head. The smallest specimen 

 on the slab is 6.5 mm. long. A specimen 7.5 mm. long has antennules which project 2.5 mm. 

 in front of the cephalon. 



Specimen No. 201 (pi. 2, fig. i; pi. 3, fig. 4). 

 Illustrated: Amer. Jour. Sci., vol. 46, 1893, p. 469, figs. 2, 3; Amer. Geol., vol. 13, 1894, pi. 3, figs. 8, 9. 



An entire specimen 17 mm. long, exposed from the dorsal side. It shows only traces 

 of the appendages of the head, but displays well those of the anterior part of the thorax, and 

 a number of appendages emerge from under the abdominal shield. This specimen is of par- 

 ticular interest as it is the subject of the first of Professor Beecher's papers on appendages 

 of trilobites. On the right side the pleura have been removed, so as to expose the appen- 

 dages of the second, third, and fourth segments from above. The first two of the appen- 

 dages on the right are best preserved, and these are the ones figured. They belong to the 

 second and third segments. The endopodites of each are ahead of the exopodites, and the 

 proximal portion of each exopodite overlies portions of the first two segments (second and 

 third) of the corresponding endopodite. The coxopodites are not visible, but very nearly 

 the full length of the first segment of the endopodite (the basipodite) is exposed. The first 

 two visible segments (the first and second) extend just to the margin of the pleural lobe, 

 while the other four extend beyond the dorsal cover. The segments decrease in length out- 

 ward, but not regularly, the meropodite being generally longer than the ischiopodite or the 

 carpopodite. The terminal segment (dactylopodite) is short and bears short sharp hair-like 

 spines which articulate in sockets at the distal end. On this specimen the anterior limb on 

 the right side shows one terminal spine, the second endopodite on that side has two, and two 

 of the endopodites on the left-hand side preserve two each. The segments of the limbs 



