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 Ccraurus, 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 Isotclus, 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 Isotclus, 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 Cryptolithus are unknown. Professor 

 Beecher in his drawing shows some fragments with 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 Kootcnia, 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, Calymene). When the axial lobe is wide, the endobases are long and stout (Isot- 

 clus. 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, Ccraurus, Kootcnia, 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 seta; 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 

 sete. The coiled branches of the exopodites as described by Walcott seenfi to me to be 

 only ordinary Triarthrus-like organs, and this, as I understand from Schuchert, was also the 

 view of Beecher. In Ccraurus 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 Ncolcnus is decidedly leaf-like, and reminds one somewhat of the exites 



