CTENOPYGB BTSULCATA. 83 



more definitely geniculate than in the latter. It is possible that they may be two 

 distinct species, but the material available is hardly sufficient to determine the 

 point, and as many other trilobites show similar variations in relative length and 

 breadth, it is probable that the difference is not specific. 



The species which is most closely allied to Ct. bisulcatais Ct. falcifcra, but there 

 are several important differences which appear to be characteristic. The glabella 

 in Ct. bisulcata is nearly parallel-sided and the posterior glabellar furrows are con- 

 tinuous across; in Ct. falcifera on the other hand the glabella narrows a little 

 forwards, and the posterior glabellar furrows are very oblique and do not meet in 

 the middle line. The eye in Ct. bisulcata is placed nearer to the posterior margin 

 and closer to the glabella than in Ct. falcifera, and accordingly the ocular ridge 

 is more oblique and the fixed cheek narrower in the former species. The axis of 

 the thorax is relatively a little wider in Ct. bisulcata and the thorax begins to 

 narrow rapidly about the fifth segment, while in Ct. falcifcra it preserves almost 

 the same width up to the eighth segment. The pleural spines are considerably 

 longer and rather more slender in Ct. bisulcata than in Ct. falcifera. 



So far as the thorax is concerned there is no danger of confounding Ct. 

 bisulcata with either Ct. pecten or Ct. expansa, for in both of these the axis is very 

 much narrower than the pleurae, while in Ct. bisulcata it is about the same width. 

 The free cheeks of Ct. bisvlcata and Ct. pecten are also easily distinguished— in the 

 former the genal spine makes a large angle with the external margin, in the latter 

 a very small angle. It is not always, however, by any means easy to distinguish 

 the cranidia. The front margin in Ct. bisvlcata is almost straight, in Ct. pecten, it 

 is emarginate. The eye in Ct. bisulcata is placed rather further back and nearer 

 to the glabella than in Ct. pecten and the ocular ridge is accordingly more oblique. 

 In Ct. pecten, moreover, the facial suture runs from the eye in a straight oblique 

 line to the posterior margin ; in Ct. bisulcata it at first runs almost parallel to the 

 margin and then curves backwards. But there are many specimens of cranidia in 

 which these characters are not clearly distinguishable. 



The thorax of Ct. bisulcata closely resembles the thorax of Splisercphthalmus 

 flagellifer as figured and described by Linnarsson. 1 It differs, however, in the very 

 much greater length of the pleural spines. Moreover, according to Linnarsson it 

 is the axis of the eighth segment in Sph. flagellifer which bears a long spine. In 

 his fig. 17, however, the spine appears to spring from the ninth segment, and in 

 Brogger's 2 figure of the same species the eighth segment shows no indication of a 

 median spine, while the ninth is broken. 



Synonymy. — It is, unfortunately, impossible to identify the actual specimens 

 upon which Phillips founded the species ; but from the form of the glabella, the 



1 Geol. Foren. Stock. Fork, vol. v (1880), p. 142, pi. v, figs. 15—17. 



2 Die Silur. Etageu 2 und 3, pi. ii, fig. 1G. 



