46 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I48 



seemingly lacking the small spine at the anterolateral angle. Orna- 

 mentation indistinct. Length of complete pygidium 2.8 mm, width 

 4.6 mm. 



Occurrence. — Collection cs-4, North Chatham. 



Discussion. — This unmistakable pygidium of Bonnia presumably 

 belongs to one of the cranidia described herein. It may be compared 

 with the pygidium of Bonnia senecta (Billings) illustrated by the 

 writer (Rasetti, 1948) which it resembles in the narrow axis and 

 well-impressed pleural furrows. 



Disposition of material. — Figured specimen : U.S.N.M. 146044. 



FAMILY, GENUS, AND SPECIES UNDETERMINED 



Undetermined pygidium No. 4 



Plate 6, figures 20, 21 



Available material. — Several incomplete examples. 



Description. — Entire pygidium subtriangular, of moderate convex- 

 ity. Axis well defined, somewhat tapered, straight-sided, almost 

 reaching the posterior margin. Axial rings numerous, probably 13 

 or 14, all well defined by ring furrows that, at least in the anterior 

 portion of the axis, show a median backward inflection. At least the 

 first few rings seem to have borne a node or spine. Pleural regions 

 downsloping, with straight, narrow, somewhat backward-directed 

 pleural furrows that, at least for the first 10 segments, correspond 

 to the ring furrows on the axis. About 11 or 12 pleural furrows are 

 visible ; there is no trace of interpleural furrows. All furrows end in 

 a broad, poorly defined border furrow that sets off a narrow, some- 

 what convex border. The anterior outline of the pleural lobes shows 

 a sharp geniculation very close to the axial furrow ; past the genicula- 

 tion the margin turns backward in a wide curve and finally assumes 

 the straight backward and inward course that gives the pygidium a 

 subtriangular shape. Surface of test smooth. The length of the 

 largest pygidium, if complete, would be about 14 mm, the maximum 

 width 12 mm. 



Occurrence. — Collection cs-4, North Chatham. 



Discussion. — This peculiar pygidium cannot be referred to any 

 described genus of Cambrian trilobites, and not even a plausible 

 family assignment is suggested. It is virtually certain that this is not 

 the pygidium of an Olenellid or Ptychoparioid trilobite, and it seems 

 unlikely that it belongs to one of the numerous Eodiscids present 

 in the collection. 



Disposition of material. — Figured specimens: U.S.N.M. 146000. 



