TRIARTHRUS. 43 



size one of the important characteristics of Triarthrus the regional differentiation of the 

 appendages. 



It should be pointed out that although Triarthrus is usually considered to be a very 

 primitive trilobite, its appendages are more specialized than those of any of the others 

 known. This is shown in their great length, the double curvature of the antennules, the 

 differentiation of four pairs of endobases on the cephalon as gnathites, and the flatten- 

 ing of the segments of the posterior enclopodites. These departures from the uniformity 

 existing among the appendages of the other genera lead one to question whether the genus 

 is really so primitive as has been supposed. 



Relation of the Cephalic Appendages to the Markings on the Dorsal Surface of the Glabella. 



Triarthrus bccki is usually represented as having four pairs of glabellar furrows, but 

 the two pairs at the front are exceedingly faint and the first of them is hardly ever visible, 

 though that it does exist is proved by a number of authentic specimens. The neck furrow 

 is narrow and sharply impressed, continuing across the glabella with a slightly backward 

 curvature. In front of it are two pairs of linear, deeply impressed furrows which in their 

 inward and backward sweep are bowed slightly forward, the ends of the corresponding 

 furrows on opposite sides nearly meeting along the crest of the glabella. In front of these, 

 near the median line, is a pair of slight indentations, having the appearance and position of 

 the inner ends of a pair of furrows similar to those situated just behind them. 



In front of and just outside this pair are the exceedingly faint impressions of the 

 anterior pair of furrows, these, as said above, being but seldom seen. They are short, slightly 

 indented linear furrows which have their axes perpendicular to the axis of the cephalon, 

 and do not connect with each other or with the dorsal furrows. The latter are narrow, 

 sharply impressed, and merge into a circtimglabellar furrow at the front. In front of the 

 circumglabellar furrow is a very narrow rounded ridge, but the anterior end of the glabella 

 is very close to the margin of the cephalon. 



Specimen No. 214, which was cleaned from the dorsal side, shows the posterior tip of 

 the hypostoma, apparently in its natural position, 3.5 mm. back from the anterior margin. 

 The entire length of the cephalon is 6 mm., so that the hypostoma reaches back slightly over 

 one half the length (0.583). The greater part of it has been cleaned off, and one sees the 

 proximal portions of the antennules, which are apparently attached just at the sides of the 

 hypostoma, 2.5 mm. apart and 2.25 mm. back from the anterior edge of the cephalon. This 

 position is distinctly within the outline of the glabella and corresponds approximately to 

 the location of the second pair of glabellar furrows. Specimens 214, 215, 216, 217, and 

 219 all seem to show the same location for the bases of the antennules. Specimen 220 is 

 the one in which the basal shafts are best preserved and the points of attachment seem to 

 be further apart in it than in any of the others. This specimen is 38 mm. long, and the 

 bases of the antennules are 5.5 mm. apart and 4 mm. behind the anterior margin. As the 

 specimen is cleaned from the ventral side, the dorsal furrows do not show distinctly, but 

 another specimen of about the same size (No. 228, 38.5 mm. long) has the dorsal furrows 

 8 mm. apart 4 mm. back of the anterior margin. 



On the same slab with specimens 209 and 210 there is an individual which, although 

 retaining the test, has had the proximal ends of the antennules so pressed against it that 

 the course of the one on the left side is readily visible. It originates in a small oval mound 



