l68 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 57 



None of the heads of the four specimens show traces of anten- 

 nules, antennae, mandibles, or maxillse. If these appendages were 

 large they have been broken off; if small they may be concealed 

 beneath the crushed and flattened large posterior section of the head. 



The thoracic legs are shown both in side view (fig. 6, pi. 27) and 

 from below on a flattened specimen (fig. i, pi. 28). They appear to 

 be of a uniform character on all the 16 somites except the two anterior 

 pairs, which may be smaller and have narrower terminal joints. The 

 legs are formed of two or three rather strong, short joints followed 

 by broad, flat, elongate-oval lobe-like joints (f, fig. 6, pi. 27). The 

 gills are shown as oval lobes on the upper portion of the leg (br, fig. 

 6, pi. 27). The terminal elongate swimming joint or fin is shaped 

 much like that of the common Branchipns vcrnalis \^errill. A strongly 

 setiferous lobe occurs above the large terminal joint, but its rela- 

 tions to it are not clear. Another feature difficult to interpret is that of 

 the groups of short, longitudinal lines shown in figure i, plate 28. My 

 present view is that they are groups of strong setae attached to one 

 or more of the lobe-like middle joints of the leg. I obtained an 

 almost similar effect by pressing flat between glass plates a specimen 

 of the recent Branchinecta paludosa (O. F. Miiller). 



The details of structure of the leg cannot be determined, but judg- 

 ing from the material available for examination they follow some- 

 what closely the leg of Thamnocephalus as illustrated by Packard.^ 



Interior structure. — The alimentary canal is readily traced from 

 the head back to the posterior portion of the terminal lobe between 

 the two points (fig. i, pi. 28). Parts of the canal are convex and 

 presumably contain portions of the matter in the canal at the time of 

 the death of the animal. 



A very beautiful specimen showing some details of the interior 

 has recently been worked out, but with the chance of getting more 

 satisfactory specimens before a more complete review of the Burgess 

 shale fauna is prepared I will not attempt to interpret its somewhat 

 confused structure. 



Dimensions. — The four specimens in the collection have the fol- 

 lowing longitudinal dimensions in millimeters : 



Length. Proboscis. Head. Trunk. Telson. 



No. I 86 24 9 44 9 



No. 2 78 16 10 45 7 



No. 3 72 20 8 37 7 



No. 4 24 9 51 Broken 



' U. S. Geog. and Geol. Surv. Territories, 12th Ann. Rept., Pt. i, 1883, pi. 14. 



