54 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES. 



are no wider than those in longitudinal sections, indicating that they were not compressed 

 but probably cylindrical. This is borne out by an individual (pi. 28, fig. 7, 1918) which is 

 not a slice but an actual specimen, the body cavity of which was hollow, and, opened from 

 above, shows the impressions of the last two coxopodites of the cephalon, and the first four 

 of the thorax. 



One transverse section (No. 63, see our fig. 15) is especially valuable, as it shows 

 the method of articulation of the coxopodites with the dorsal skeleton. Another specimen 

 (No. 73) shows that appendifers are present in Calymene, and while the appendifer does 

 not retain its original form in slice No. 63, the section does show clearly that there was a 

 notch in the inner (upper) side of the coxopodite into which the lower end of the appen- 

 difer fitted, thus giving a firm, articulated support for the appendage. This notch appears 

 to be slightly nearer the outer than the inner end of the coxopodite, and since it must have 

 made a kind of ball-and-socket joint, considerable freedom of movement was allowed. The 

 appendage must have been held in place by muscles within the coxopodite and attached to the 

 appendifer. 



No slice which I have seen shows a continuous section through all the segments of an 

 endopodite, but many, both longitudinal and transverse, show one, two, or as many as three 

 segments. 



Such sections as No. 120 show that the endopodites of the thorax were slender and 

 composed of segments of rather uniform diameter. Other sections, notably No. 83, 154, 

 and in, show that they tapered distally, and bore small spines at the outer end of each 

 segment. 



The exopodites of course furnish the chief difficulty in interpretation. Doctor Wal- 

 cott finds two sets of structures attached to the coxopodite, a long, slender, spiral exopo- 

 dite, and a short, broad epipodite with a fringe of long setae. Since he has given the same 

 interpretation for Calymene, Ceranrus, and Acidaspis, I have considered the question of 

 all three together on a preceding page (p. 48), and given my reasons for regarding both struc- 

 tures as due to sections in different directions across setiferous exopodites. 



Sections like those shown in figures n, 13, and 14 of plate 27 (1918) happen to be cut 

 in or near the plane of the setae of an exopodite, and so show hairs of considerable length. 

 Such sections are, as would be expected, very few in number, while sections like those shown 

 on figures 4, 5, 7, and 9 of plate 27, which cut the setae more nearly at right angles, are 

 very common. Slices which give any definite idea of the form of the shaft of the exopodite 

 are exceedingly rare. Perhaps the most satisfactory one is No. 23 (pi. 3, fig. 3, 1881), 

 which shows the proximal part of a long, slender, unsegmented shaft, with the bases of a 

 number of slender setae. The organ is not complete, as would be inferred from the pub- 

 lished figure, but the section cuts diagonally across it, and the total length is unknown. 

 It is directed forward, like the exopodites of Ncolcnus, but whether or not this is a natural 

 position is yet to be learned. 



The proximal, non-setiferous portion of the exopodite is evidently at an angle with 

 the setiferous part. Another similar exopodite is apparently shown by specimen 29 (pi. 3, 

 fig. 9, 1881, which has a similar angulated shaft and just a trace of the bases of the setae. 



Pygidial Appendages. 



That appendages were present under the pygidium is shown by longitudinal sections, 

 but nothing is known of the detail of structure. 



