156 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES. 



trate the convergent ridges on the inside of the ventral membrane in the axial region of the 

 thorax. These ridges are very low, and on each segment of the thorax there is a central one, 

 outside of which is a pair which are convergent forward, making angles of 35 to 45 

 with the axis. 



The metastoma is shaped much like the hypostoma of an Illanus. It is convex, nearly 

 semicircular, with the straight side forward, and there is a continuous raised border around 

 the curved sides and back. This border is separated from the central convex body by a deep 

 linear depression. 



The hypostoma is also rather well preserved and has a narrow, slightly elevated border 

 at the sides and back. 



Measurements: The incomplete specimen, from which only a very small portion of the 

 length is missing, is 9 mm. long. The metastoma is 0.45 mm. long and 0.58 mm. wide. 



Specimen No. 214 (pi. i, fig. 2; pi. 3, fig. 6). 



This is a large specimen, developed from the ventral side. It shows the antennules and 

 some other appendages of the head, but derives its special interest from the excellent pre- 

 servation of a few of the exopodites, which are turned back parallel to the axis of the body 

 and lie within the axial lobe. 



The shaft of the exopodite is made up of numerous short segments which at their an- 

 terior outer angles are produced into spines, and which also bear movable spines along the 

 anterior border. As shown in several other specimens, the exopodite ends in a more or less 

 long spoon-shaped segment bearing on its lower surface a broad groove. No set;e appear 

 to be attached to this, but both anterior and posterior margins bear numerous small, appar- 

 ently movable spines. From the groove along the ventral side of the remainder of the 

 exopodite arise numerous long slender filaments which become progressively shorter toward 

 the tip. This specimen shows that they are not cylindrical, but are flattened along opposite 

 faces, at least at their distal ends. While no connection can be seen between adjacent setae, 

 they seem to stay together like the barbs on a feather. 



Measurements: Length, 33 mm., width at back of cephalon, 16 mm.; from front of 

 cephalon to back of hypostoma, 6 mm. 



Specimen No. 219 (pi. 2, fig. 6; pi. 4, fig. 4). 

 Illustrated: Amer. Jour. Sci., vol. 13, 1902, pi. 4, fig. i, pi. 5, fig. 4 (photograph and drawing) > 



The endopodites of most of the appendages of the thorax are well shown, and occasional 

 portions of exopodites. The coxopodites are long, flattened, and do not taper much. The 

 anterior and posterior edges of the basipodites of the endopodites of the first two segments 

 are approximately parallel, but on the succeeding endopodites the basipodites and ischiopo- 

 dites are triangular in form, with the apex backward. In successive endopodites toward 

 the posterior end, the angle made by the backward-directed sides of the basipodites becomes 

 increasingly acute, so that in some of the posterior appendages this segment is wider than 

 long. The ischiopodite shows a similar increase of width and angularity on successive seg- 

 ments, and the meropodites and carpopodites also become wider on the posterior segments, 

 and even triangular in outline toward the back of the thorax and on the pygidium. 



Along the median portion of the axial lobe the specimen has been cleaned until the inner 

 side of the ventral membrane was reached. Here the test shows on the inner surface at each 

 segment of the thorax a series of low ridges which are roughly parallel to the axial line, but 



