232 



genal angles very clearly. Still, it is most probable that A. Susce is 

 nothing more than a broad, short form of the Isotelus gigas of De Kay, 

 the Asaphus platycephalus of Canadian and some United States palaeon- 

 tologists. The writer can see practically no difference between Isotelus 

 Suscn, as figured by Dr. Clarke on page 708 of the " Lower Silurian Trilo- 

 bites of Minnesota," and Asaphus platycephalus, as previously figured by 

 E. Billings in the "Geology of Canada" and in the "Catalogues of the 

 Silurian Fossils of the Island of Anticosti." 



A glabella collected at Jjittle Black Island by Mr. Tyrrell in 1889, 

 which is four inches in length but imperfect laterally, and very broadly 

 rounded in front, may have formed part of a very large specimen of 

 A. Susce. 



Asaphus (Isotelus) gtgas, De Kay. 



? Asaphus platycephalus, Stokes 1822. Trans. Geol. Soc. Lond., sei". 2, vol. 



I., p. 208, pi. 27. 



Isotelus gigas, De Kay 1824. Ann. Lye. Nat. Hist. N. York, vol. 



I., p. 174, pi. 12, fig. 1, and pi. 13, fig. 1. 



Green 1832. Mon. Trilob., pp. 67 and 68. 



Vanuxem 1842. Geol. Rep. N. York, p. 46, fig. 1. 



Emmons 1842. /i., p. 389, fig. 1. 



Conrad 1843. Proc. Ac. Nat. So. Philad., vol. I^ 



pp. 330 and 331. 



Hall 1847. Pal. N. York, vol. I., p. 231, pis. 60, 



figs. 7, a-i ; 61, figs. 3, a-m, and 4, a-c ; 62, 

 figs. 1, a-c ; and 63. 



.1 ,1 Emmons 1855. Amer. Geol., vol. I., pt. 2, p. 215, pi. 



16, fig. 12. 



? Asaphus platycephalus, Billings 1863. Geol. Canada, p. 184, fig. 183, and p. 



218, fig. 229. 



, 1866. Geol. Surv. Canada, Gat. Silur. Fobs. 



Isl. Anticosti, p. 24, fig. 7. 



? Asaphus platycephalus, 'BiWmgs 1870. Quart. Journ. Geol. Surv. Lond., vol. 



XXVI., p. 479, pi. 31, figs. 1-6, and pi. 32, 

 figs. 1 and 2. 



Isotelus gigas. Miller 1877. Amer. Pal. Foss., first ed., p. 219. 



Asaphus gigas. Miller 1889. N. Amer. Geol. and Palseont., p. 531. 



Isotelus gigas, Clarke 1894. Lower Silur. Trilob. Minn., p. 701. 



Dr. John M. Clarke, who has recently studied an extensive series of 

 specimens of Isotelus gigas and /. maxvmus (megistos) from the Trenton 

 limestone of the state of New York, says that he is convinced that the 

 only valid distinction between these forms is the absence of cheek spines 

 in the one and their presence in the other. He also adopts the name 

 Isotelus gigas in preference to Asaphus platycephalus, on the ground that 

 " none of the figures given by Stokes shew the structure of the genal 

 angles, and it is therefore wholly a matter of presumption whether his 

 specimens were of the same character as those afterwards fully described 

 and illustrated by De Kay." (Op. cit. supra., p. 706.) 



