236 TRILOBITA CHAP. 
pygidium radiate from it in a fan-lhke manner. Occasionally, as 
in Bumastus, the axis cannot be distinguished from the lateral 
parts. In a few early Trilobites (Olenellus, Holmia, Fig. 148, 
Paradouides, Fig. 147) the lateral parts of the pygidium are 
very small. In some genera, such as Asaphus, the marginal part 
of the pygidium forms a flattened or concave border. The 
margin may be entire or produced into spines, and sometimes 
(Fig. 151, C) a caudal spine comes off from the end of the axis. 
On the ventral surface of the pygidium there is a marginal rim 
sunilar to the doublure of the cephalhe shield. The anus is on 
the ventral surface of the last segment of the pygidium. 
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! showed their existence 
in American specimens of Asaphus megistos, Calymene senaria, 
and Cheirurus pleurezacanthus. 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 Trinuclews, 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.” 
In 7riarthrus 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, 
1 Bull. Mus. Comp. Zool. Harvard, viii., 1881, p. 191. 
2 Studies in Evolution, 1901, pp. 197-225 ; Geol. Mag. 1902, p. 152. Walcott, 
Proc. Biol. Soc. Washington, 1x., 1894, p. 89. 
