L917] 



The Ottawa Naturalist. 



77 



WAS THE LOWER CAMBRIAN TRILOBITE SUPREME?* 



By Lancaster D. Burling. 



A new species of Lower Cambrian trilobite, Paedeumias robsonen- 

 sis, has recently been described"*" from a single specimen which is 

 unique among the thousands of specimens of known trilobites in its 

 imperfection. The reason for the extraordinary perfection of the tests 

 of previously discovered trilobites has apparently escaped critical 

 observation though the late development of the ability to roll up into a 

 ball has been appealed to as indicating that the trilobite was the 

 supreme arbiter of the early Cambrian seas and needed no such pro- 

 tection. He has recently been deposed from this position, however, at 

 least for the Middle Cambrian, and his title conferred upon Sidneyia 

 inexpert mis WalcottL a Merostome-like crustacean which has been 

 described as armed with a truly formidable set of chelate appendages 

 or claws (see figure 2). In the Lower Cambrian, however, nothing 

 has hitherto been discovered that would dispute the claim of the 

 trilobite to be the largest inhabitant of the seas, and the specimen of 

 Paedeumias above mentioned is as large or larger than any other 

 fossil so far discovered in the lower Cambrian rocks of the North- 

 western Cordillera. It is further noteworthy in having the greatest 

 number of ribs (44) ever discovered in a trilobite. Fifteen of these 

 are of large size and their long spines almost completely encircle the 

 remaining 2 ( ) (or more, the end is broken off) which are smaller and 

 nearly equal in size. 



~"nS 



Fig. 1 Paedi uraias robson urling. Outline of right half of the first 



seven ribs, showing extent of injury. As can be seen by the photo- 



t on plate I, Vol. XXX, opp. p. 53, the left half of 



i ibs is normal, with the exception that the fifth is slightly 



shorter than the fourth anil sixth. The enlargement of the third rib 



is characteristic of the family to which the species belong. Tl 



fourths natural size. 



"Published by permission of the Deputy Minister of Mine 

 tOttawa Naturalist, vol. 30. 1916, pp. 53-58, pi. I. 

 JNai. Geog. Mag., vol. 22, 1911, p. 511. 



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