THE JOURNAL 



OF THE 



i^uchett Pur0St0jjual aClxtt^ 



On British Graptolites. By John Hopkinson, F.R.M.S. 

 (Read December 18tk, 1868.) 



In one of the oldest series of rocks, the Silurian, a peculiar group 

 of fossil zoophytes, called Graptolites, occurs. Thej are exclu- 

 sively confined to the Silurian formation, and in its lower division 

 are frequently unaccompanied by any other fossils. They are by 

 far the most abundant in the Skiddaw slates and Llandeilo flags, 

 lessening in number, both of species and individuals, in the Upper 

 Silurian rocks, until, in the Ludlow, but two species are found. 

 Their geological range is thus comparatively very limited ; but their 

 geographical range is world-wide. In Britain, on the Continent, 

 in America, and in Australia ; in fact, wherever Silurian rocks 

 occur, graptolites abound. 



To what class of zoophytes they belong, whether to the Hydrozoa, 

 the Actinozoa, or the Polyzoa (Bryozoa), is not easily determined. 



Sir Roderick Murchison says, " They are supposed by many 

 naturalists to have been zoophytes nearly allied to the living Vir- 

 gularia, a creature known only in deep water. Others rather con- 

 eider these extinct forms to belong to Sertularian zoophytes, or even 

 to Polyzoa. Be this as it may, the geologist has observed that 

 they are found exclusively in the Silurian system of life." 



As microscopists, however, we need not enter further into the 

 geological or geographical range of graptolites, but consider instead, 

 their structure, history, classification, and affinities. 



