44 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRTLOBITES. 



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. ii. Triarthrus becki 

 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. u). 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 I mm. wide and 0.5 mm. long, and the entire trilobite is 11.5 mm. long. 



