236 TRILOBITA 



CHAP. 



pygidium radiate from it in a fan-like manner. Occasionally, as 

 in Bitmastus, the axis cannot be distinguished from the lateral 

 parts. In a few early Trilobites (Olenellus, Holmia, Fig. 148, 

 Paradoxides, 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 

 similar to the doublure of the cephalic 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 pleurexacanthns. 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 



01 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. 2 



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 carl) side of the hypostome, and have the structure of antennae, 



1 Bull. Max. Comp. Ziiol. Jliirvdi-iJ, viii., 1881, p. 191. 



2 Studies in Evolution, 1901, pp. 197-225; Gcol. May. 1902, p. 152. Walcott, 

 Proc. fiiol. Soc. Washington, ix., 1894, p. 89. 



