FOSSIL HYDROZOA. 



163 



described by Dr Allman in the singular Polyzoon, EJidbdo- 

 pleura. 



From the above definition, it will be seen that the Grapto- 

 lites agree with the living Sertularians in possessing a cor- 

 neous polypary, which not only invests the ccenosarc, but is 

 expanded into little cups or " hydrothecae," within which 

 each polypite is protected. The Graptolites, however, differ 

 from the Sertularians in the fact that the polypary was 

 unattached, and apparently free-floating, whilst it has not, 

 except in a few cases, anything like the plant-like appear- 

 ance of the latter. Further, the hydrothecse of the Grapto- 

 lites, except in the genus Rastrites, 

 always more or less overlap one 

 another; whereas those of the Ser- 

 tularians are not in contact. Lastly, 

 no Sertularian exhibits any structure 

 which can be compared with the 

 " solid axis " of the Graptolites. 



Taking such a simple Graptolite 

 as Monograptus priodon (fig. 54), or 

 M. Sagittarius (fig. 55, A), as the 

 type of the sub-class, the polypary 

 is seen to consist of three elements, 

 which are known as the " solid axis," 

 the " common canal," and the " cell- 

 ules." The " solid axis " is a cylin- 

 drical fibrous rod which gives support 

 to the corneous and flexible polypary. 

 The term " solid " is probably a mis- 



nnrnpr fnr it waa alrnnat novtoiTilir 



, was almost certainly 



hollow, and filled With living mate- 



. 



rial. It appears to be absent in 

 Retiolites Geinitzianus, but some un- 

 certainty rests upon this point. As 

 a very general rule, it is prolonged 

 as a longer or shorter naked rod 

 beyond one or both ends of the poly- 

 pary, and either extension may be more or less dilated. Its 

 basal prolongation, with or without an accompanying exten- 



Fig. 54. Morphology of Mono- 

 graptus priodon. A., Monograptus 

 priodon, Bronn, preserved in relief 

 lateral view slightly enlarged; 

 B, Dorsal view of a fragment of 

 the same species considerably 

 enlarged ; c, Front view of a 

 fragment of the same, showing 

 the mouths of the cellules much 

 enlarged ; D, Transverse section 

 of the same. All from the base of 

 the Coniston Flags. (Original.) 



