2 BRITISH CAMBRIAN TRILOBITES. 



pressing duties, have now extended over a period of many years, I have received 

 so much kindly assistance in the loan of specimens and in other ways that it would 

 not be possible here to mention the names of all those who have helped me. But 

 to Dr. Henry Woodward I am especially indebted for his continued encouragement 

 in the study of a branch of science which he has made his own ; and to Prof. 

 T. McKenny Hughes for the facilities which he has afforded to me at Cambridge ; 

 while abroad my debt is greatest to one whom now I cannot thank, the late Prof. 

 Lindstrom, of the Vetenskaps Akademi in Stockholm. 



In the present instalment I am also particularly indebted to Mr. Frank Raw, 

 B.Sc, of the University of Birmingham, for allowing me to quote his descriptions 

 of Agnosttt.s dux and Agnostus callavei, together with his notes on their affinities. 

 Mr. Raw has made a special study of the Shineton Shale fauna, including these 

 two forms, but his results have not yet been published. 



Because of the imperfect condition of so many of our Cambrian fossils I have 

 drawn special attention, wherever it seemed necessary, to those peculiar character- 

 istics of each species which are least likely to be obscured in a poorly preserved 

 specimen, and which at the same time serve to distinguish it from other forms 

 with which it might be confused. These notes, which are added immediately after 

 the formal description, will, I hope, prove useful to geologists who desire to 

 identify species for stratigraphical purposes and who may have to depend entirely 

 upon fragmentary and distorted specimens. It should, however, be borne in mind 

 that species which are not closely allied to each other may possess a superficial 

 resemblance which is likely to survive the effects of compression and distortion ; 

 and I am sometimes compelled to compare forms which really differ widely from 

 one another, and which could not possibly be confounded if they were perfectly 

 preserved. 



SYSTEMATIC DESCRIPTIONS. 



Family Agnostid^. 



Genus AGNOSTUS, Brongniart. 



The genus Agnostus has been divided by Tullbcrg into four sections, which are 

 characterised as follows : 



(1) Longifrontes. — This group is distinguished by the prominent glabella and 

 pygidial axis, which are usually fairly long. The test is either smooth, or the 

 cheeks are furrowed, or the test both of cheeks and tail is ornamented with 

 raised points. The margin is usually narrow. The cheeks in front of the glabella, 



