XIII. NOTES ON ORDOVICIAN TRILOBITES : ILLiENID^E 



FROM THE BLACK RIVER LIMESTONE NEAR 



OTTAWA, CANADA. 



By Percy E. Raymond and J. E. Narraway. 



Introduction. 



This is one of a series of papers on the trilobites of the Ordovician, 

 and deals with some new or little known species of Illaenidae from the 

 Lowville and Black River Limestones at Ottawa, and a few related 

 species from the Chazy and Trenton Limestones. 



The material on which this study is based is a large collection 

 obtained by Mr. Narraway from the Ordovician formations about 

 Ottawa. Mr. Narraway has been fortunate in obtaining many entire 

 specimens of species which are rare, so that we are able to supplement 

 the description of some species previously known only from fragments. 



Whole specimens of trilobites are very rare, and for material for 

 comparative study, we have been obliged to draw upon the resources 

 of some of the older museums. 



This survey of the material has shown that in the past the boundaries 

 of some of the species have been somewhat loosely drawn, and several 

 forms with a general resemblance have been placed in the same species. 

 While such a course is one of great convenience, especially to the stu- 

 dent who wishes to identify the ordinary imperfect material in his 

 collection, it is a source of confusion when certain kinds of work are 

 undertaken. This lax identification of species makes particularly diffi- 

 cult the study of zoological provinces and the distribution of isolated 

 or connecting basins in the Paleozoic seas. As an example, take the 

 case of Bumastus milleri Billings. At Ottawa there occurs a distinct 

 group of trilobites of the genus Bumastus, all very much alike and all 

 having nine segments. To call these animals Bumastus milleri invites 

 attention to this distinctive characteristic, but to include these trilobites 

 under the name Bumastus trentonensis with other fossils from New 

 York having from eight to ten segments and with forms from Minne- 

 sota having eight or nine segments, obscures the evidence which might 

 be obtained from them. All the forms are closely related, and a 



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