1916] The Ottawa Naturalist. 5 5 



somewhat justified in assuming that our trilobite lost this por- 

 tion of his anatomy to a foe more voracious, if not larger, than 

 himself. The occurrence certainly lends weight to the infer- 

 ence that the Lower Cambrian trilobite was not the supreme 

 arbiter we have supposed him to be ; however, he may have been 

 struck by material dislodged from a ledge beneath which he 

 was crawling. 



Horizon and Locality. Lower Cambrian, Mahto forma- 

 tion, collected from drift block on the slope of the Mural glacier 

 just under Mumm Peak, Mt. Robson region, British Columbia. 

 Collected by E. C. Amies. 



Observations. Paedeumias robsonensis differs from P. 

 transitans the only other species known (a): (1) in having 

 29 instead of 2 to 6 rudimentary segments posterior to the spine- 

 bearing fifteenth, and in the more highly developed character 

 of the rudimentary segments they are better described as 

 small short ribs in P. robsonensis, while those of P. transitans 

 are truly rudimentary; (2) in having a highly ornamented 

 cephalic surface; (3) in the character of the glabellar furrows. 

 which approach closely to those represented in the cephalon 

 from Mt. Stephen, B.C., doubtfully referred to Olenellus giU 

 berti by Walcott (b) ; and (4) in the width and flatness of the 

 marginal cephalic rim. 



Paedeumias robsonensis resembles P. transitans in general 

 shape and outline, in the number and general character of the 

 normal segments and the pleural furrows, and in the abrupt- 

 ness of the change from regular to rudimentary segments. 



These resemblances seem to warrant the inclusion of 

 Paedeumias robsonensis in the genus Paedeumias, but the 

 differences are such as to justify its reference to a new species. 

 The specific name robsonensis is derived from Mount Robson, 

 in whose general vicinity this trilobite was secured. 



General Considerations. The resorption of segments 

 posterior to the fourteenth or fifteenth in the genera of the 

 Mesonacidae (a family of trilobites apparently confined to the 

 upper portion of the Lower Cambrian) would seem to indicate 

 that the functioning parts, those necessary for the life of the 

 individual, were confined to the first fourteen. Once this adap- 

 tation to fourteen vital segments is made, and Wanneria appears 

 to show the trilobite at the moment this took place, resorption 

 of the remainder begins. The finding of 29 rudimentary pos- 

 terior segments would seem to indicate that this resorption 

 takes place laterally, that is, they become smaller in size before 



(a) Walcott, Smithsonian Misc. Coll., vol. 53, 1910, pp. 30S-310, pis 24, 25, 32, 33, 34 and 



44. 

 (6) Smithsonian Misc. Coll., vol. 53, 1910, pi. 36, fig. 16. 



