202 BULLETIN OF THE 
Plate IV. fig. 5, and Plate VI. fig. 1, show it in a normal position as 
attached to the frontal “doubling” of the head. The structural ele- 
ments are essentially the same for each species. In the sections shown 
in Plate I. figs. 1-10, it is cut across at various points, and also at 
different angles to the longitudinal axis, which, combined with its 
concavo-convex form and recurved margin, causes the sections of it 
to vary greatly.in outline. Longitudinal sections are shown in the 
sections illustrated on Plate V. figs. 1— 4. 
Careful search has been made for traces of an antennal system, but 
thus far without success. In one section a delicate jointed appen- 
dage occurs near the hypostoma. It has been frequently examined, 
but it is still unidentified, as from its structure it cannot be a fragment 
of an antenna. 
The fifth conclusion, given in the “Notes on some Sections of Trilo- 
bites from the Trenton Limestone," * is, that the mouth is posterior 
to the hypostoma, and consists of four pairs of manducatory jaws, 
formed by the basal joints of the four anterior pairs of appendages. 
With the exception of a slight modification of the first part, this con- 
clusion may be permitted to stand as expressing our present knowl- 
edge of these parts. The mouth is not strictly posterior to the 
hypostoma, but is a little above and between it and the anterior end of. 
the median lobe of the thoracic membrane, opening obliquely back- 
ward, instead of directly downward. 
The four pairs of appendages have a general structure similar to 
the cephalic legs of Limulus and Hurypterus. The basal joints are 
larger than the others, and undoubtedly subserved the function of 
manducation. No one leg or appendage has been seen entire, but from 
several sections, Plate I. figs. 6-10, and others not illustrated, each 
leg is found to be formed of either six or seven joints. The basal 
joints of the three anterior pairs of legs are smaller than those of the 
fourth pair, and have their anterior or proximal end obliquely truncated. 
as shown by the section represented in Plate I. fig. 6. The remain- 
ing joints are slender, and not unlike those of the thoracic legs. The 
basal joints of the fourth pair are more than twice as long as broad, 
and have the posterior inner angle cut away so that the anterior por- 
tions alone approximate to form a part of the manducatory apparatus. 
From the distal end a comparatively slender joint extends to unite with 
one or two succeeding joints, which support several more expanded joints 
that form what is considered a swimming leg. These are shown in 
* 31st Rep. N. Y. State Museum, p. 63, 1879. 
