J. HOPKINSON ON BRITISH GRAPTOLITES. 153 



Of graptolites nothing is preserved besides the polypary — ^the 

 cfenosarc, poljpites, and tentacles having perished. 



The reproductive organs of the Hydrozoa consist usually of 

 portions of the ectoderm developed into pouches or sacs, termed 

 gonopJioi'es. These are always external, generally budding from 

 the ca^nosarc, to which, or to the protecting periderm, they are at 

 first attached. In the Sertulariadje these gonophores, or genera- 

 tive buds, are sometimes developed directly from the caenosarc, and 

 sometimes from the gonohlastidium, a peduncle, which rises from 

 caBUOsarc and has a similar structure. The gonophores are some- 

 times contained in cup-like receptacles, termed gonothecce. The 

 entire reproductive organs, whatever be their nature or form, are 

 included under the general term gonosome. 



These are bat a few of the terms specially relating to the 

 Hydrozoa, yet they are more than sufficient for the graptolite. 

 There is, however, one structure peculiar to the GraptolitidEe, and 

 that is, a slender solid axis, supporting the hydrosome in every 

 direction. 



We shall now easily understand the structure of the graptolite.* 

 The proximal end, or initial point, which appears seldom to have 

 been attached to foreign bodies, consists of a radicle, and is some- 

 times invested with a corneous disc. The radicle is sometimes long 

 and slender, sometimes broad and tapering, and sometimes only a 

 minute point. It seems to be merely the proximal termination of 

 the solid axis. In many graptolites a short spine is developed on 

 each side of the radicle, and sometimes, though very rarely, the 

 radicle itself bifurcates. The corneous disc is only present in some 

 of the branching forms, uniting the proximal terminations of each 

 branch. It consists of two membranes, and may have contained a 

 substance of the same fleshy nature as the cjenosarc. Its purpose 

 seems to have been to give strength and rigidity to the polypary. 



Between the radicle and the first developed hydrothec^, in many 

 branching forms, a non-polypiferous portion of the periderm inter- 

 venes. Hall has called it the funicle, but as it is analogous to 

 the hydrocaulus of recent Hydrozoa, no new term is needed. This 

 hydrocaulus differs from the rest of the periderm, only in being 

 composed of a thicker membrane, and containing a smaller centraF 

 canal. 



* All the points of structure and varieties of form here referred to are illus- 

 trated in Plate VIII. 



K 2 



