44 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES. 
whose posterior margin impinges upon the third glabellar furrow near the middle of its 
course, and just outside the outer end of the second glabellar furrow. The cephalon of this 
specimen is 5 mm. long, and the point of origin of the left antennule is 2.75 mm. in front 
of the posterior margin and 0.75 mm. from the dorsal furrow. 
It is therefore evident that the antennules in this species are not attached beneath the 
dorsal furrows, but within them and opposite the second pair of glabellar furrows. 
All cephalic appendages behind the antennules are attached somewhat within the dorsal 
furrows, the first pair as far forward as the antennules and the last pair apparently under 
the anterior edge of the neck ring. They do not appear to correspond in position to the 
posterior glabellar furrows and neck ring, being more crowded. The last pair is attached 
to appendifers beneath the nuchal segment, and the first pair beneath the third glabellar fur- 
rows. There are no depressions on the dorsal surface corresponding to the points of at- 
tachment of the mandibles. 
Anal Plate. 
Professor Beecher, during his first studies of Triarthrus, found no appendages pertain- 
ing to the anal segment, but later evidently came upon a spinose anal plate which he caused 
Fig. 11. — Triarthrus beclti 
Green. Anal plate of specimen 
65525 in the U. S. National 
Museum. Drawn by Doctor 
Wood. X 20. 
to be figured. The specimen (No. 201) on which this appendage is preserved is cleaned 
from the dorsal side, and the anal plate is a small, bilaterally symmetrical, nearly semicir- 
cular structure margined with small spines. Specimen 202 also shows the same plate (pi. 5, 
fig. 6), but it is imperfectly preserved. It has a large, perforation in the anterior half. 
Both of these specimens are in the Yale University Museum. 
The anal plate is especially well shown by specimen 65525 in the United States National 
Museum (fig. 11). This specimen is from Rome, New York, and two photographs of it 
have been published by Walcott (1918, pi. 29, fig. 6; pi. 30, fig. 19). It is developed from 
the dorsal side, and the anal plate is displaced, so that it projects behind the end of the 
pygidium. It is semicircular in shape, with a hemispheric mound at the middle of the an- 
terior half. Two furrows starting from the anterior edge on either side of the mound 
border its sides, and, uniting back of it, continue as an axial furrow to the posterior mar- 
gin. The mound is perforated for the opening of the posterior end of the alimentary canal. 
The lateral borders of the plate bear five pairs of short, symmetrically placed spines. The 
plate is 1 mm. wide and 0.5 mm. long, and the entire trilobite is 11. 5 mm. long. 
