Compared 

 with other 

 species. 



Comparative 

 abundance. 



228 



Size. Length of the middle piece of the head shield 3 mm. width, 5 

 mm, Length of the movable cheek, 3 mm. ; width, exclusive of the genal 

 spine, 2 mm. Width of the genal spine, 2$ mm. ; length, 25 mm. Length 

 of the pygidium, 1 mm. ; width, 2 mm. Length of a young hypostome, 

 1 mm. ; width, 1 mm. 



Locality and horizon. Limestone bed in C. 36 at McAdam's shore, 

 Escasoriie, Cape Breton, N.S. 



This form is distinguished from the mutation Canadensis of S. alatus, 

 found in the upper Cambrian shales at St. John,* and from the type of 

 that species, by its long, flat and very wide, falcate genal spine. It 

 agrees nearly with a spine and free cheek figured by Linnarsson, but 

 not referred to any species. The cheek portion of S. Fletcheri is very 

 small compared with the spine, which is stiffened by the two sharp 

 ridges that run along the middle ; these ridges occupy about a third of 

 the width of the spine, the rest being flat. 



The pygidium of this species differs from that of the type of S. alatus 

 as figured by Linnarsson in the possession of narrow side lobes (about as 

 wide as the marginal fold) ; that author's figure gives no side lobes, the 

 marginal fold being in contact with the rachis.f 



For numbers, this is the dominant species in the trilobite bed at McAdam 

 shore, as will be seen by the following proportion of forms found on five 

 square inches of surface of one of the layers. 



Sphaerophthalmus Fletcheri , 30 heads. 24 cheeks. 



Agostus trisectus, chiefly the mut. ponepunctus 9 " 1 pi 6 pyg. 



Ctenopyge pecten 8 " 3 cheeks. 



Peltura scarabeoides .... 1 " 4 pi. 1 pyg. 



Parabolina Dawsoni . . , 1 " 



Sphaeroph- 

 thalmus 

 alatus. 



All the heads of Sphserophthalmus were not counted ; several were so 

 small that the generic characters were not well shown. 



SpHjEROPHTHALMUS ALATUS, Boeck. 



1838. Trilobites alatus, Boeck, Gsea. Norveg., p. 143. 

 1848. Olenus humilis, Phill. Mem. Geol. Surv., vol. ii., pt. i, p. 55, figs. 

 4, 5, 6 and p. 347. 



* Trans. Roy. Soc. Can. voL xi, sec. iv, p. 107, pi. xvii, figs, lla and 6, 12a and 6. 

 f Geol. forening, i Stockholm Forhandl, 1880 No. 60 Bd. No. 4, p. 2 fig. 14. 





