IÇTÔ. No. lO. THE LOWER CAMBRIAN HOLMIA FAUNA. 45 



Still higher the genus is found in the Protolenus fauna which can be 

 assumed to belong to the youngest part of the Lower Cambrian. In this fauna 

 CoBBOLD^ found the interesting Sfr. Sa/opiensis Cob. in Shropshire, and 

 possibly also other species. The youngest Stroiuella forms are an undeter- 

 mined species from the Paradoxides intcrmedius fauna in Shropshire, which 

 about corresponds with the Tessiiii zone, and also Str. depressa Grønwall'^ 

 from the Davidis- zone in Bornholm. 



The genus thus extends from the Lower Cambrian up to the middle 

 part of the Middle Cambrian, but the main extension may be said to be in 



f^'g« 5- <*• Almost perfect specimen of Streniiella stroma Bil. after Shimer y- \\. h. pygidium 

 of Strenuella strenua BiL, after Walcott. c. pN-gidium of Streuuella sp. after Moberg. b-c 



are strongly enlarged. 



the upper Lower Cambrian and the older part of the Middle Cambrian. 

 The Middle Cambrian species are confined to the Scandinavian and English 

 regions. 



Although we know a considerable number of species of this genus, 

 our knowledge of their structure is still very limited. As regards the 

 majority of them, indeed, only cranidia without the free cheeks have hit- 

 herto been found, and it is only in the case of one or two species that 

 we have certain knowledge of the structure of thoracic segments and p3'gidium. 



The most complete specimen of a Strenuella that has hitherto been 

 found was described by H. W. Shimer from the Lower Cambrian at Mill 

 Cove, North Weymouth, Massachusetts and belongs to Billing's variet}' 

 Str. strenua (Text. fig. 5a). It shows the cranidium with free cheeks and 

 9 segments. The free cheeks are narrow, and extend as short, broad 



' E. S. CoBBOLD. „On Small Trilobites from the Cambrian Rocks of Comley". (Quart. 

 Journ. Geol. Soc. Vol. 66, 1910, p. 31.1 



^ E. S. CoBBOLD. „The Trilobite Fauna of the Comlej' Breccia-Bed. (Quart. Journ. Geol. 



Soc, Vol. 69, 1913, p. 32 ) 

 ^ Bornholms Paradoxideslag. (Damn. Geol. Unders. II. Ræk. No. 13. 1902, p. 159.1 



