1916. No. 10. THE LOWER CAMBRIAN HOLMIA FAUNA. 47 



unusually broad free cheeks and the long spines on the occipital ring. The 

 free cheeks that are referred to the above form, in contrast with Str. stretiua 

 Bill, have long, curved genal spines. The segments have a marked EUipso- 

 cephalide form, with the marked geniculation of the pleura and well-defined 

 oblique furrows. The rachis rings have either long vertical spines, or only 

 quite short ones, and the pleura extend into short, deflected spines; presu- 

 mabl\- this applies especially to the central ones, whilst those in front are 

 more evenly pointed. The pygidium is small, and strongly resembles that 

 described by Walcott for Sfr. stroma, var. nasnta Walcott. The axis 

 however, is smooth without furrows, is still broader, and extends right out 

 to the posterior margin. The p^'gidium is not known in the case of the 

 Norwegian torms, but on the other hand I have been able to describe a 

 number of segments that in structure closely resemble those described by 

 CoBBOLD. The tiny deflected spines of the pleura, too, could be distinctly 

 demonstrated. Median spines however, were not found. 



As will be seen, no Strennella species is known in complete specimens 

 nor can any be reconstructed with full certainty, for several structural 

 features are still unknown. Thus we are still uncertain as regards the 

 number of segments. Shimer believes that in Û\e Cd^sç oï Strenuella strenua 

 Billings there were scarcely more than 12 segments. However, this is not 

 probable. It is true that Ellipsocephalus Hoffi Schlothei.m has only 12; 

 but the older and completely known E. polytomus Linxarsson has 14, and 

 the same number is seen also in E. German Bar. Since in the case of 

 the Mcsonacidac and other Cambrian families, the number of segments may 

 be assumed to have undergone a reduction in the course of phylogenetic 

 development, in the case of the old Strcnuella forms the number can scar- 

 cely have been less than 14; probabh' there were considerably more. 



Remarkably enough, several of the body parts appear to have varied 

 greatly in form. This is the case with the free cheeks, the rhachis and 

 pleural spines and also the pygidium. 



As regards the free cheeks, they are known in the case of two species 

 only, Strctifiella strenua Billings and Salopiensis Cobbold. In the former 

 the}' end in short, broad spines, in the latter in long and narrow ones. 

 As the development that can be demonstrated in the Ellipsocephalidae ends 

 in a complete reduction of the genal spines, we must assume the last 

 mentioned form to be the most primitive of the two. 



This is confirmed also by a consideration of the form of the segments 

 in the case of StrenucUa strenua Billings, for according to Shimer's 

 description, the pleura are short and pointed, nearl}' as in the case of 

 Ellipsocephalus Hoffi Schlothei.m, whilst in E. Salopiensis they extend into 



