- SUB-CLASS I TRILOBITA 615 
Most of the recent advances in the knowledge of Trilobite structure have 
come from the study of numerous very perfectly preserved specimens of 
Triarthrus Becki, Green, from the Utica Slate (Ordovician), near Rome, New 
York. Undoubted antennae in this form were discovered by Valiant, and first 
announced by Matthew in 1893. Subsequently a series of papers was 
published by Beecher on the detailed structure of this Trilobite, which is now 
Say 

Fic. 1267. 
Triarthrus Becki, Green. Utica Slate (Ordovician); Rome, New York. 
A, Dorsal, and B, Ventral aspect, 
2/) (after Beecher). 
the best known of any species, and necessarily forms the basis of much of the 
following summary of ventral organs. 
In the median line anteriorly, there is first the hypostoma or upper lip, at 
the end of which, and opening obliquely backward, is the mouth (Walcott, in 
Calymmene). In Triarthrus the lower lip, or metastoma, is a convex arcuate 
plate, just posterior to the extremity of the hypostoma. At the angles on 
either side are two small elevations, or lappets. 
Paired Appendages.—All segments of the cranidium, thorax, and pygidium, 
except the anal segment, carry paired appendages, which are‘all biramous save the 
anterior pair. The anterior antennae, or antennules, are attached at the sides 
of the hypostoma, and consist of a simple, many-jointed flagellum (Fig. 1267). 
