The endopodites of the thorax of Triarthrus, Cryptolithus, and Acidaspis show progressive modification from front to back in the broadening of the individual segments and the assumption by them of a triangular form. Not only do the individual segments become more triangular from front to back, but more of the segments of each endopodite become triangular. This modification has so far been seen in these three genera only. The individual segments, except the distal ones, seem to be flattened in all these genera. The distal end of the terminal segment of each endopodite of Triarthrus bears three small movable spines, and each of the segments usually bears three or more spines, located in sockets along the dorsal surface and at the anterior distal angle of each segment. The endopodite of Cryptolithus is bent backward at the carpopodite and this segment is always thickened. At the distal end of the dactylopodite there is a tuft of spines, the triangular segments have tufts of spines on their posterior corners, and there are groups of spines also in the neighborhood of the articulations.
The endopodites of Ceraurus, Calymene, and Isotelus are all relatively slender, the segments are parallel-sided, and there seems to be no particular modification from front to back of the thorax. The endopodites of Isotelus are short, the entire six segments of one being but little longer than the coxopodite of the same appendage. The segments of the endopodites of Neolenus are mostly short and wide, and at the distal end of the terminal segment there are three stout spines. In Kootenia the endopodites are long and very slender. The endopodites of Ptychoparia are too poorly preserved to show details, and those of the thorax of Acidaspis likewise reveal little structure, but they seem to have the triangular modification, and to turn back somewhat sharply at about the position of the carpopodite.
Beecher showed that in Triarthrus there was a pair of appendages on the pygidium for every segment of which it is composed except the last or anal segment (protopygidium). Walcott has since shown that in Neolenus this segment bears a pair of cerci, and Beecher's drawings show that in his later studies he recognized a spinous plate, the possible bearer of cerci, on the anal segment of Triarthrus. The appendages of the anal segment have not yet been seen on other species of trilobites.
The appendages of the pygidium do not show any special modifications, but seem in all cases to be similar to those of the posterior part of the thorax. In Cryptolithus all the pygidial appendages are short and remain beneath the cover of the dorsal test, while in Triarthrus and Neolenus they extend behind it.
In the latter genus the endopodites of the pygidial appendages appear to be practically identical in form with those of the thorax, the individual segments being perhaps a little more nearly square in outline. Like those of the thorax, the segments of the pygidial endopodites bear numerous short spines. The caudal cerci are richly segmented, slightly flexible, spinous tactile organs. They are symmetrically placed, nearly straight when in their natural position, and make an angle of about 75 with one another. They appear to be attached to a narrow rim-like plate which seems to fit in just ahead of the doublure of the pygidium, or perhaps over it.
In Ceraurus, Calymene, and Isotelus, the endopodites of the pygidium are similar to those of the thorax, but seemingly more slender, with less well developed coxopodites, and with, in the last-named genus, slender cylindrical segments. Exopodites are not known on the pygidia of any of these genera, but since they are present and like those of the thorax in Triarthrus, Cryptolithus, Neolenus, and Ptychoparia, there is little reason to think that they were absent in Ceraurus or Calymene, though there is some question about Isotelus.
The limbs are largest and longest on the anterior part of the thorax of a trilobite, and diminish regularly in length and strength to the posterior end of the pygidium. This regular gradation shows, as Beecher was the first to point out, that the growing point of the trilobites is, as in other arthropods, in front of the anal segment. New free segments are introduced into the thorax at the anterior end of the pygidium, and this has led to some confusion between the growing point and the place of introduction of free segments.
If a new segment were introduced at a moult in front of the pygidium, that segment would probably have less fully developed appendages than those adjacent to it, and so make a break in the regular succession. The condition of the appendages corroborates the evidence derived from the ontogeny of the pygidium, and proves that the new segments are introduced at the same growing point as in other Arthropoda.