110 Proceedings of the Royal Physical Society. 



St David's, were detected by myself in the Lower Palaeozoic 

 rocks of Girvan, Ayrshire. They occur here in extraordi- 

 nary abundance, and apparently in a good state of preserva- 

 tion. The examples collected showed at once that in this 

 genus we are no longer dealing with a compou-nd symmetrical 

 siculate Graptolite, like Ccenogra^ptus gracilis, H., but with an 

 extraordinary thread-like fossil, forming an irregular mesh- 

 work, and occurring in unbroken nets, probably several feet 

 in diameter. The filiform branches appear not to be strictly 

 confined to a single bed, but pass irregularly through a slight 

 thickness of the rock. Even in the calcareous shales, where 

 the other organisms are preserved with their full relief, few 

 of these branches show any appreciable thickness. Where 

 they are best preserved they appear to be cylindrical. As in 

 the American and Welsh forms, there is nothing that can be 

 interpreted as the remains of hydrothecm or cellules beyond 

 an irregular ill-defined crenulation of the margins of the 

 branches. These fossils rarely occur in association with the 

 true Graptolites, but when they are so associated, the dis- 

 tinctness of the two is placed beyond question, as these net- 

 like forms, even when best defined, show nothing of that 

 carbonaceous character so marked among the Graptolitidce. 

 The thread-like branches vary in colour, from dark grey or 

 green to purple, according to the varying composition and 

 texture of the matrix. They are generally bordered with a 

 strong discolouration, about one-tenth of an inch in width. 

 This feature, however, is not invariable, and in rare cases is 

 altogether absent. 



I have recognised similar fossils in the Bala rocks of 

 North and South Wales, in the Skiddaw slates of the Lake 

 District, and in many localities among the Silurian rocks of 

 the south of Scotland. It may be regarded as certain that 

 they are widely scattered through the Lower Palaeozoic 

 sediments ; but that their unsatisfactory state of preservation 

 and enigmatical character has hitherto caused them to be 

 neglected by the palaeontologist. 



The Girvan species appears to be restricted in its range to 

 a definite formation, and it may be expected that other species 

 will eventually be found to possess a certain geological value. 



