CTENOPYGE. 79 



referring other forms to this genus, Linnarsson based his opinion sometimes on the 

 width of the hinder end of the thorax, sometimes on the width of the fixed cheeks. 

 In retaining Angelin's Splia&rophthalmus fiagellifer in Sphssrophthalmus he seems to 

 have been guided chiefly by the rapid narrowing of the thorax, for according to 

 the character of the fixed cheeks the species should belong to Ctenopijge. 



Brogger * came to the conclusion that the tail of Ctenopijge pecten, as described 

 by Linnarsson, is not the true pygidium in the same sense as the tail of Sphaeroph- 

 thalmus. He states that the segments are fused only at the axis and that the 

 pleurae are free. He looks upon these pleurae as really a part of the thorax, and 

 suggests that the small terminal portion behind the pleurae is the true homologue 

 of the tail of Sphserophthalmus. He further points out that while heads of Ct. 

 bisulcata and of other forms referred to Ctenopijge have been found in great abun- 

 dance, no tails in any way similar to that of Ct. pecten have been discovered with 

 them. He concludes that the tails of these species must be small or they could 

 not have been overlooked, and he infers that the large many-spined tail cannot be 

 a generic character, and probably there was no great difference between the tails 

 of Gtenopyge and those of Sphavrophthalmus. He therefore bases his definition of 

 Ctenopijge entirely upon the characters of the head and thorax. According to him 

 the head is usually very transverse, with very long cheek-spines ; the hinder thoracic 

 pleurae are geniculate and produced into long reflexed spines. He refers Sphsero}>h- 

 thalmus flagellifer, Ang., to Gtenopyge. 



Moberg 2 has attempted to confute Brogger's arguments and concludes that 

 Linnarsson's view is correct. He shows that the tail of Ct. pecten is quite distinct 

 from the thorax, but he does not deal with the characters of the tail in the other 

 species referred to the same genus. 



Our English specimens confirm Moberg's conclusion. The tail of Ct. pecten, as 

 defined by Linnarsson, is certainly not a part of the thorax. It is commonly 

 found detached, and evidently the segments of which it is formed were firmly 

 united to one another. Brogger and Moberg agree that only the axial rings are 

 fused while the pleurae are free ; but this is not what our numerous specimens seem 

 to indicate. The pleurae seem to be closely connected and rarely show any signs 

 of separation. The small unsegmented plate at the end of the axis is clearly not 

 the homologue of the tail of SpJiasrophthalmus. 



But whether this peculiar form of tail occurs in other species is doubtful. The 

 hinder end of the thorax in Ct. bisulcata and Ct. falclfera is comparatively narrow, 

 and the tail cannot have been very large. The small specimen of Ct. bisulcata 

 figured in Plate IX, fig. 4, even seems to indicate that the tail was entire as in 

 Sphserophthalmus ; but the specimen is too imperfect to admit of any certain con- 

 clusions being drawn from it. 



1 Die Silur. Etagen 2 im.l 3, p. 114. 



- Geo!. Foren. Stockh. Fork, vol. xiv (1892), p. 351, 



