6G Silurian Fomis of Canada* 



of the eye lialf the length of the ghihella including the neck seg- 

 ment. There is a "svide concave groove around the base of the 

 eye outside and a narrow one behind. The cheeks outside of 

 the groove are smooth sh'ghlly convex and with no marginal 

 furrow. 



The thorax consists of eleven segments, the axis semi-cylindri- 

 cal gently tapering backwards, about one third the wdiole width 

 of the thorax when the pleuiss are curved in their natural posi- 

 tion but one fourth less than the length of a pleuron straightened 

 out. The fulcrum or bend of the pleurae is distant from the 

 axis about one half the width of latter. 



The pygidium is broadly rounded behind with six or seven 

 ribs on the prominent conical axis and four or five broad flat 

 indistinct ones each divided a'ong tlie middle in the lateral lobes. 

 The ribs on the axis become more and more indistinct back- 

 wards and those on the sides extend scarcely two thirds from it 

 thus leavinsf a broad smooth maroin all round. 



The posterior extremity of the axis is not well preseryed in any 

 of the specimens that I have seen and I have not therefore ascer- 

 tained whether or not it is well defined or confluent with the gene- 

 ral surface. The surface appears to be smooth but as the best 

 specimens seem to have been a little worn it may be that the 

 glabella is tubercular. It is difiicult to get accurate measure- 

 ments from rolled up specimens, but the following appear to be 

 early the proportions as shewn in two individuals. 



No. 1. No. 2. 



Length of head 3 4 lines. 



" of thorax 6 ? 8 ? Hues. 



" of pygidium . . 2-^ 3 lines. 



Total Hi 15 



The width of the head measured between the posterior angles 

 is about twice the leno^th. 



This species somewhat resembles P. SloJcesli (Milne Edwards) 

 but that species has the lens-bearing surface of the eye broadest 

 behind while in ours it is broadest in front. There are from six 

 to nine lenses in the vertical rows of P. Stohesii but in Phacops 

 Orestes only five at the most. 



Locality and formation. — Middle Silurian Anticosti and Gasp©. 



Collectors, — J. Richardson, Sir W. E. Losran. 



