THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES. 



similar to those along the anterior margin. The gnathobases are but slightly modified to 

 serve as mouth parts, much less so than in Triarthrus, but the coxopodites of the cepha- 

 lon are shorter and wider than those of the thorax. 



At the distal end of the coxopodite arise the endopodite and exopodite. The endo- 

 podite consists of six segments, the distal ones, propodite and dactylopodite, more slender 

 than the others, the last bearing three terminal spines. The first endopodite is shorter than 

 the others and slightly more slender (pi. 16, fig. i) 1 and the anterior appendages turn 

 forward more or less parallel to the sides of the hypostoma (pi. 22). The basipodite, 

 ischiopodite, meropodite, and carpopodite are, in their flattened condition, roughly rectan- 

 gular, only a little longer than wide, taper gradually distally, each bears small spines on the 

 outer rim, and some of the proximal ones usually have a row along the margin. 



The exopodites of the cephalon, as of the body of Ncolcnus, are very different from 

 those of any other trilobite whose appendages were previous!)' known. As shown in the 

 photographs (pi. 20, fig. 2; pi. 22), each exopodite consists of a single long, broad, leaf- 

 like blade, not with many segments as in Triarthrus, but consisting of a large basal and 

 small terminal lobe. It bears on its outer margin numerous relatively short, slender, flat 

 setae. The long axes of the exopodites point forward, and the setae are directed forward 

 and outward. They stand more nearly at right angles to the shaft on the cephalic exopo- 

 dites than on those of the thorax. This same type of broad-bladed exopodite is also found 

 on the thorax and pygidium. 



The number of functional gnathobases on the cephalon is unknown. That four endo- 

 podites were present on one side is shown pretty clearly by specimen 58591 (pi. 16, fig. 3) 

 and while no more than two well preserved exopodites have been seen on a side, there 

 probably were four. Specimen 65513 (pi. 16, fig. i) shows gnathobases on the second and 

 third appendages of that 'individual as preserved, but there is no positive evidence that these 

 are really the second and third appendages, for they are obviously displaced. The hypos- 

 toma of Neolenus is narrow but long, several specimens showing that it extended back to 

 the horizon of the outer ends of the last pair of glabellar furrows. It is not as \vide as the 

 axial lobe, so that, while gnathobases attached beneath the first pair of furrows would prob- 

 ably not reach back to the posterior end of the hypostoma, they might lie parallel to it and 

 not extend beneath. It seems possible, then, that there were four pairs of endobases but that 

 the second rather than the first pair served as mandibles, as seems to be the case in 

 Ceraurus. 



Thorax. 



The thorax of Ncolcnus consists of seven segments, and the appendages are well shown 

 (pi. 17, fig. i; pi. 18, figs, i, 2; pi. 20, fig. i.), The endopodites of successive segments 

 vary but little, all are slender but compact, and consist of a long coxopodite with six short, 

 rather broad segments beyond it. In the figures, the endopodites extend some distance in 

 a horizontal direction beyond the edges of the dorsal test, as many as four segments being 

 in some cases visible, but measurements show that the appendages tended to fall outward on 

 decay of the animal. The dactylopodites are provided with terminal spines as in Triarthrus. 

 The coxopodites are long, straight, and slender. They are well shown on only one speci- 

 men (pi. 18), where they are seen to be as wide as the basipodite, and the endobases are set 

 with spines on the posterior and inner margins. They are so long that those on opposite 



1 Nota bene! All references in this section are to the plates of Doctor Walcott's paper in 1918. 



