66 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES. 
function as mouth-parts (gnathites), and are especially modified for this purpose in Triar- 
thrus, being flattened, shoe-shaped in outline, and so arranged that they work over one an- 
other in a shearing fashion. While the more anterior of the coxopodites are attached in 
front of the posterior tip of the hypostoma, the gnathites of Triarthrus bend backward so 
that all are behind the hypostoma. In Calymcne and Ceraurus, two or three pairs of the 
gnathites are back of the hypostoma, and one or more pairs may be beside or under the 
hypostoma. In these genera the mouth is probably in front of the tip of the upper 
lip. In IsoteluSj the mouth seems to have been situated in the notch between the 
two branches of the hypostoma, and the gnathites of two or three pairs of the appendages 
probably worked under its forks. Since the length of the hypostoma differs in the various 
species of Isotelus, there would be a variable number of gnathites projecting under its forks, 
according to the species. In this genus the gnathites are of the same long form, cylindrical 
in cross-section, as the endobases of the thoracic segments, but each is bowed back consider- 
ably from the point of attachment. 
The gnathites of Ncolcnus are like the endobases of the thorax, but broader. The great 
length of the hypostoma makes it probable that the mouth was far back and that some of 
the gnathites were in front of it. The gnathites of Crypt olithus are unknown. Professor 
Beecher in his drawing shows some fragments Avith toothed ends near the hypostoma, and 
it may be that they are inner ends of gnathites, but I see nothing to substantiate such an in- 
terpretation. If, as some suppose, Cryptolithus was a mud feeder, the gnathites were prob- 
ably poorly developed. Of the gnathites of Kootenia, Ptychoparia, and Acidaspis also 
nothing is known. 
Thorax. 
In each genus there is a pair of appendages for each segment of the thorax. When 
the axial lobe is narrow, the endobases of the coxopodites are small and short (Cryptolithus, 
Ceraurus, Calymcne). When the axial lobe is wide, the endobases are long and stout (Isot- 
elus. Triarthrus) . The exopodites always lie above and in front of the corresponding endop- 
odites. In Triarthrus the two branches are of practically equal length. In Cryptolithus the 
exopodites are much the longer. In Neolenus, Calymcne, Ceraurus, Kootenia, and Pty- 
choparia, the exopodites are shorter than the endopodites. 
The exopodites in Triarthrus consist of a proximal shaft, succeeded by numerous short 
segments, and ending distally in a long, grooved, somewhat spatula-shaped segment. Along 
the anterior margin of the shaft there are many small spines. Along the posterior margin 
there are numerous flattened seta: which all lie in one plane and which seem to be more or 
less united to one another like the barbs of a feather. The setae are short, not much longer 
than the width of one of the thoracic segments, and point backward and outward. In Cryp- 
tolithus the shaft does not seem to be made up of small segments, and is narrow, with a 
decided backward curve. The setae are considerably longer and much more flattened than 
in Triarthrus. In Calymcne the state of preservation does not allow a very full knowledge 
of the exopodites, but they appear to have a slender, unjointed shaft and short and delicate 
setaa. The coiled branches of the exopodites as described by Walcott seem to me to be 
only ordinary Triarthrus-like organs, and this, as I understand from Schuchert, was also the 
view of Beecher. In Ceraurus the exopodite seems to have been somewhat paddle-shaped, 
expanded at the distal end, and to have had rather thick, blade-like setse. 
The exopodite of Neolenus is decidedly leaf-like, and reminds one somewhat of the exites 
