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 JEurypterus. 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. G. 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. 



