Graptolites. 371 



their whole course ; polypary reticulated on tlie outer surface. 

 Two species of this singular genus have been found in the 

 Wenlock beds of Britain (Plate I., Fig. 12). 



Section III. — Species with single and double series of cells 

 on different parts of the same polypary. 



11. Bicranograptus , Hall. This is proposed by Hall as a 

 subgenus of Glimacograptus ; but as the form of the polypary 

 has been used by all authors as the principal basis for the 

 separation of genera, this must be recognized as a good genus. 

 A single species, G. ramosus, Hall, has only hitherto been 

 described in Britain, and that is from the Llandeilo beds. It 

 is figured (13). 



Section IV. — Species with four series of cells. 



12. Phyllograptus, Hall. Polypary consisting of four 

 laminae joined throughout their whole length to a common 

 solid axis, and so giving four separate and independent sets of 

 cells. A single species has been found in the Llandeilo rocks, 

 but I have figured (Plate I., Fig. 5, a) an American species and 

 (5, b) a transverse section, after Hall, which exhibits, better 

 than the more imperfectly preserved British specimens, the 

 structure of the genus. 



Systematic position. — There is, perhaps, no small group of 

 fossils concerning which so many and so different estimates 

 as to their systematic position have been entertained. Linnaeus, 

 as we have seen, considered that the species which he knew 

 was not a true fossil. Bromel, as early as 1727, is believed to 

 refer to graptolites in his account of the fossils of Sweden, 

 when he speaks of the fossil leaves of grasses ; their vegetable 

 origin has been maintained by several subsequent writers. 

 Brongniart includes them among the Algae, and figures two 

 species in his great work, Histoire des Vegetaux Fossiles ; and 

 in this opinion he was followed by several of the earlier 

 American geologists, as Mather, Conrad, and Vanuxem. 



As equally erroneous, the opinions of Boeck, M'Crady, and 

 Nimmo may be at once set aside. Boeck considers them to 

 have been hollow tubes, rent asunder in different ways before 

 being buried in the mud in which they are preserved. They 

 were probably, he thinks, the arms of radiata or cephalopoda, 

 and the various forms described as different species are the 

 result of the accidental tearing, and the irregular contraction 

 of the substance of the tube. M'Crady considers that the 

 graptolites are the larva) of cchinoderms, because of their 

 similarity in form to Muller's published drawings. Nimmo 

 thinks they are nothing more or less than the serrated spines 

 of the Raja pastinaca, or an allied species. It does not appear 



