236 TRILOBITA 



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

 in Btimastus, the axis cannot be distingnished from the lateral 

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

 Paradoxides, Fig. 147) the lateral parts of the pygidiuni are 

 very small. In some genera, such as Ascqjhus, the marginal part 

 of the pygidiuni 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 pygidiuni 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 pygidiuni. 



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 pygidiuin. 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 

 2datycep]ialus. Soon afterwards Walcott ^ 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 pygidiuni ; 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 tlie appendages. Subsequently extended specimens 

 or Triarthrus (Fig. 142) and Trinucleus, showing the ventral 

 surface and appendages clearly, were discovered in the Utica 

 Slate (Ordovician) near Eonie, New York. A full account of 

 the appendages in those specimens has been given by 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, 



1 BiM. Mus. Comp. Zool. Harvard, viii., 1881, p. 191. 



" Studies in Evolution, 1901, pp. 197-225 ; Geol. Mag. 1902, \). 152. Walcott, 

 Proc. Biol. Soc. Washinfjton, ix., 1894, p. 89. 



