AET. 11 SOME BURGESS SHALE FOSSILS HUTCHINSON" 5 



specimen 57683 are much retouched. After considerable study of all 

 the material I was unable to detect more than 15 appendage-bearing 

 somites and believe that the first segment figured by Walcott is 

 really jjart of the head and does not bear a leg. 



Abdomen. — The abdomen consisting of a broad spatulate lobe is 

 apparently divided into two parts. The larger anterior part has a 

 concave posterior edge produced at the side into a pair of spines, 

 between which the much smaller posterior division lies. The anus 

 opens on the posterior part of the latter. The structure of this 

 region is well shown in specimen 56784. (Walcott 1912, pi. 28, fig. 

 1.) The large anterior part is clearly a true segment; the status 

 of the posterior part must remain doubtful. 



Appendages. — Considerable diversity in appearance is exhibited 

 by the trunk appendages of different specimens. This is probably 

 due to the position of fossilization rather than to any actual differ- 

 ences implying a mixture of species. The lateral aspect of the trunk 

 appendages can best be studied in specimen 57683. In this specimen 

 the first 14 are flat leaflike lobes hanging down at the side of the 

 body. (Fig. 1.) Each appendage apparently lies somewhat in 

 advance of the segment bearing it, so that the upper anterior margin 

 slopes obliquely upward and backward to the insertion which is 

 marked by the remains of musculature. Walcott said that the 

 appendages were jointed, but I can find no trace of joints, nor would 

 such be expected in a foliaceous appendage. Attached to the outer 

 surface he also describes " gills " and a " strongly setiferous lobe " 

 on the distal part of the appendage. The supposed gills in Walcott's 

 figure are marks made by irregular splitting of the shale and no 

 trace of them is to be found either in the well-preserved anterior 

 appendages in specimen 57683 or in other relatively perfect speci- 

 mens. The anterior part of the limb is, however, somewhat dilated 

 in the former specimen, but the convexity is not separated by any 

 suture or joint from the limb itself. The " strongly setiferous lobe " 

 presumably refers to the whole posterior part of the distal end of 

 the appendage. This area in 56783 is clearly demarcated from the 

 rest of the appendage and may represent a flabelluni. It is, how- 

 ever, not certainly detectable in other specimens. The setae are 

 probably illusionary; I can detect no certain setae on the external 

 surface of any limb. 



The internal edge of the foliaceous appendage carried a series of 

 very strong thick setae. (Fig. 25.) These are visible in both well- 

 preserved dorso-ventrally compressed specimens and in various frag- 

 ments. In all cases they are exposed by the breaking away of the 

 outside flat surface of the appendage. The latter was presumably 

 considerably curved in frontal section, the concavity being directed 



