400 THE AM KMC AN NATURALIST . [Vol. LIV 



known material preserving- the ventral anatomy. There- 

 fore it seems to me that Walcott has not sufficient evi- 

 dence for the structures he illustrates and describes as 

 epipodites and exites in Xeolcuus, epipodites in Triar- 

 thru.s, Calymene, and Ceraurus, and spiral gills in the 

 last two. 3 The presence of none of these things can, in 

 my view, be proved. 



There is little modification of the appendages of differ- 

 ent parts of the body. The gnathobases of the coxopo- 

 dites on the cephalon of Triarthrus are more jaw-like 

 than those on the remainder of the body, and in the same 

 species the segments of the endopodites of the pygidium 

 and posterior part of the thorax are more triangular than 

 those of the anterior ones. Cryptolithus has the thoracic 

 legs bowed backward to form more efficient pushing or- 

 gans, and in all species with long hypostomata the an- 

 terior biramous appendages seem to be more or less 

 degenerate. Thus, Calymene appears to have two pairs 

 of very delicate biramous appendages back of the anten- 

 nules, and the first one or two pairs of gnathobases of 

 Calymene, Ceraurus and Neolenus seem to be somewhat 

 reduced, but there is no evidence that any pair of ap- 

 pendages is entirely lost. All the evidence seems to 

 indicate that Beecher correctly homologized the cephalic 

 appendages with tlie antennules, antennae, mandibles, 

 maxillulae, and maxillae of the Crustacea. 



Form of Body 

 Trilobites are always depressed, flattened animals, 

 with a broad head composed of at least five fused seg- 

 ments, a thorax of from two to forty-four free segments, 

 and a pygidium made up of a variable number of undif- 

 ferentiated segments. The anal opening is at the pos- 

 terior end of the pygidium, and the growing point just 

 in front of it, as in other arthropods. New segments are 

 introduced into the posterior end of the pygidium during 

 moults, are pushed forward by the introduction of others 



•Smithsonian MiseL Coll., 1918, Vol. 67, No. 4. 



