Note on Leptoplastus. 461 



Note on Leptoplastus. 



By G. F. Matthew, M.A., F.R.S.C. 



In the number of this journal for October, 1889, the author 

 communicated a short paper on the " Occurrence of Lepto- 

 plastus in Acadian Cambrian Eocks," and referred two 

 species of trilobites to that genus. 



Since then, on studying the geological range of a trilobite 

 which occurs with these two, he was led to see that there 

 was a discordance between the range of the two species 

 (supposing them to be Leptoplasti), and that of the third 

 one — Agnostus pisifcrmis, L. In Sweden this species belongs 

 to the base of the Olenus-bearing strata; but a variety 

 (socialis, Tullberg) is found in the middle of the Olenus 

 beds, and an allied species, A. cyclopyge, Tull., as high as 

 the layers containing Parabolina spinulosa. 



The range of this species of Agnostus (A. pisiformis) and 

 its relatives in Sweden is thus below the horizon of Lepto- 

 plastus. Moreover, the Acadian form of Agnostus pisiformis, 

 by its narrower pygidial rachis and other features, appears 

 to be a somewhat more primitive form than the type of the 

 species found so many years ago in Sweden, and therefore 

 possibly older. 



The locality in the Kennebecasis valley where these three 

 trilobites were found was for this reason re-examined, and 

 additional parts of the species were found. The additional 

 material did not bear out the reference of the two species to 

 Leptoplastus. Soon after a species of trilobite found in the 

 St. John Basin, undoubtedly of the genus Leptoplastus, 

 removed any doubt there might have been that these species 

 were wrongly referred. 



Angelin says of the movable cheek of Leptoplastus that 

 it is compressed. This is true of the two Acadian species 

 in question, but he evidently meant compressed all around ; 

 in these Acadian trilobites, however, the cheeks are com- 

 pressed on the front and back only, and at the genal angle 

 run out into a rather long spine ; this was fouud to be 



