yi BRITISH FOSSILS. 



value on account of the abundant details of structure, which, through 

 our extensive collections, we are enabled to delineate ; others, as 

 Ogygia Portlockii, Calymene tuberculosa, and Ampyx nudus, are critical 

 species, the history of which has been hitherto imperfectly known ; and 

 a third class consists of entirely new forms, such as Illcenus Davisii 

 and Olenus micrurus ; the last mentioned trilobite has great interest, 

 having been found in the most ancient fossiliferous strata of the Silurian 

 System, whilst at the same time it constitutes a good representative of 

 the numerous forms of the genus Olenus, which occur in the oldest 

 rocks of the Continent. 



The descriptions of all the species, with the exception of Ampyx nudus, 

 have been drawn up by Mr. Salter, whose attention has for some time 

 been specially directed to Trilobites. 



Illustrations of the other British genera of Trilobites are in pre- 

 paration. 



EDWARD FORBES. 



July, 1849. 



