Although Trilobites are often found in abundance and in an excellent state of preservation, it is only in very rare cases that anything is seen of the ventral surface except the hypostome and the reflexed borders of the cephalic shield, of the thoracic segments, and of the pygidium. The usual absence of =appendages= is probably due to their tenuity. Billings, in 1870, first obtained clear evidence of the presence of pairs of appendages, in _Asaphus platycephalus_. Soon afterwards Walcott[191] showed their existence in American specimens of _Asaphus megistos_, _Calymene senaria_, and _Cheirurus pleurexacanthus_. In the two latter species the appendages were found by cutting sections of curled-up specimens obtained from the Trenton Limestone; 2200 examples were sliced, of which 270 showed evidence of the existence of appendages. They were seen to be present on the head, thorax, and pygidium; a ventral uncalcified cuticle with transverse arches was also found. By means of sections of curled-up specimens it was difficult to determine satisfactorily the form and position of the appendages. Subsequently extended specimens of _Triarthrus_ (Fig. 142) and _Trinucleus_, showing the ventral surface and appendages clearly, were discovered in the Utica Slate (Ordovician) near Rome, New York. A full account of the appendages in those specimens has been given by Beecher.[192] [Illustration: FIG. 142.—_Triarthrus becki_, Green, × 2½. Utica Slate (Ordovician), near Rome, New York. =A=, Ventral surface with appendages; _Ep_, metastome; _Hy_, hypostome. =B=, second thoracic appendage; _en_, endopodite; _ex_, exopodite, × 12. (After Beecher.) ] In _Triarthrus_ each segment, except the anal, bears a pair of appendages, all of which, except the first, are biramous. There are five pairs of cephalic appendages; the first pair are attached at each side of the hypostome, and have the structure of antennae, each consisting of a single flagellum formed of short conical joints. The other cephalic appendages increase in size successively. At present the second and third pairs are not satisfactorily known, but appear to have been similar to the fourth and fifth pairs. The second pair is attached at the level of the posterior end of the hypostome. The fourth and fifth pairs have large, triangular coxopodites which served as gnathobases, their inner edges being denticulate; the endopodites consist of stout joints; the exopodites are slender, and bear setae which are often arranged in a fan-like manner.