Wiltshire fossil in the frontispiece, different in its structure from all 

 the rest, is formed of a spongeous substance, which is pervaded and 

 embraced by ramifying tubuli or fibres. 



Thus, if the figure of the fossil be assumed as the leading character 

 of the species, subtances, differing materially in their structure, will 

 be classed together in the same species; and, on the other hand, if the 

 species be formed on the external structure, we shall have, under the 

 same species, substances differing as widely in their forms as the fos- 

 sils at Plate XI. Fig. 6 and 7. 



LETTER XIV. 



ALCYONITES INCLOSED IN FLINTS FOUND IN WILTSHIRE, OX- 

 FORDSHIRE, BUCKINGHAMSHIRE, AND HERTFORDSHIRE OTHER 



SPECIMENS NOTICED, 



IT is to my truly learned and respectable friend, the Rev. Mr. Towns- 

 end, that I owe the favour of having my attention first called to the 

 fossils, the investigation of which I shall next attempt. That gentle- 

 man, not only obliged me with a sight of his own beautiful drawings, 

 but also presented me with a specimen of hydrophanous flint, which, 

 Avith many other more perfect specimens, he had himself found in the 

 Vale of Pewsey. These he shewed me were evidently full of changed 

 organized matter, belonging to some unknown species of zoophyte : 

 of which he pointed out to me, not only their internal parts, but even 

 their cortical investments. 



These flints were ofanoblong,andsomewhat of a rounded form ; they 

 seeming to differ from the common black flint, in having one of their 



