Fig. 19. Restoration of a transverse section of the thorax of Ceraurus pleurexanthemus Green, showing the relation of the appendages to the appendifers and the ventral membrane. The probable positions of the heart and alimentary canal are indicated.
No section shows anything which can be identified as any part of the antennules, so that these organs have been supplied from analogy with Triarthrus.
There are undoubtedly four pairs of biramous Cephalic appendages, but their points of attachment are not so obvious. There are two pairs of conspicuous appendifers on the posterior part of the cephalon and another pair almost concealed by the hypostoma. It is probable that the appendages of the cephalon were not attached directly beneath them, as the four pairs have to be placed within the space occupied by the three pairs of appendifers. As the mouth is in front of the posterior end of the hypostoma, the gnathites of the first pair of biramous appendages may have extended beneath that organ, or they may have lain beside it, and only become functional when the hypostoma was dropped down in the feeding position. The second pair of gnathites reached just to the tip of the hypostoma, and the other two pairs seemingly curved backward behind it.
The points of attachment on the thorax, as shown clearly in sections, were directly beneath the lower ends of the appendifers. The endopodites were long enough to reach to or a little beyond the outer extremities of the pleural spines, while the exopodites were apparently somewhat shorter. Each endopodite consisted of six short, fairly stout segments, each with at least two spines on the somewhat expanded distal ends. The exact form of the exopodites could not be made out. The shaft was apparently rather short, unsegmented, and fairly broad. The setæ appear from the sections to have been more or less blade-shaped and to have overlapped, as do those of the exopodites of Cryptolithus. Judging from their position in the sections, the setæ not only bordered the posterior side of the shaft, but radiated out from the end as well.
The pygidium shows three pairs of functional appendifers, hence three pairs of appendages have been supplied. There is a fourth pair of rudimentary appendifers, but as they are beneath the doublure they could not have borne ambulatory appendages.
The Appendages of Acidaspis trentonensis Walcott.
A single individual of Acidaspis trentonensis, obtained from the same locality and horizon as the specimens of Triarthrus and Cryptolithus, when cleaned from the ventral side shows a number of poorly preserved endopodites which seem very similar in shape and position to those of Triarthrus. One endopodite on the right side of the head and the first five on the right side of the thorax are the best shown. All are slender, are directed first forward at an angle of about 45 with the axis, then, except in the case of the cephalic appendage, turn backward on a gentle curve and extend a little distance beyond the margin of the test, but not as far as the tips of the lateral spines of the thoracic segments.