298 CRUSTACEA. TRILOBITA 



way, and it tapers more rapidly than the axis of the 

 thorax ; in Bronteus it is very short. The margin of the 

 pygidium may be even or entire, or may be provided with 

 a posterior spine or with lateral spines. This margin is 

 bent under so as to form a border on the ventral surface 

 similar to that on the ventral surface of the head. 



For a long time the appendages of the Trilobites were 

 unknown. In the great majority of specimens, when the 

 under surface is exposed, the only parts which are found 

 to be preserved are the hypostome and the reflexed borders 

 of the dorsal exoskeleton. But in rolled-up specimens of 

 Galymene and Cheirurus, Walcott showed, by means of thin 

 sections, that jointed appendages are present on the head, 

 thorax and pygidium, and that the ventral surface of the 

 body is formed of a thin, uncalcified cuticle, strengthened 

 by transverse arches. 



More recently specimens in which the body is not 

 rolled up, showing clearly the ventral surface with the 

 appendages, have been obtained from the Utica Slate 

 near Rome (New York), and have been fully described by 

 Beecher. The most important of these belong to the 

 genus Triarthrus (fig. 128). Each segment of the body, 

 excluding the last (or anal), is found to bear one pair of 

 appendages, which, with the exception of the first, are 

 biramous. On the head there are five pairs of append- 

 ages. The first are the long antennae which consist of a 

 large basal joint and numerous short conical joints and 

 are attached on each side of the hypostome (h); these 

 appear to be the only appendages in front of the mouth, 

 and are considered by Beecher to represent the anten- 

 nules (or anterior antennae) of other Crustacea. The 

 remaining four pairs of appendages of the head are 

 biramous and all appear to have nearly the same form but 



