162 



CCELENTERATA. 



tervals. Each branch " is formed of a series of articulations 

 marking the position of minute cells" (E. Forbes). Old- 

 hamia has been variously referred to the Sertularian Zoo- 

 phytes, to the Polyzoa, and to the vegetable kingdom. The 

 most probable conjecture, perhaps, would refer the genus to 

 the calcareous sea-weeds (Salter). 



III. SUB -CLASS GRAPTOLITIM (Graptolites EHABDO- 

 PHORA, Allman). The Graptolites form a very large and 



important family of fossils which 

 usually present themselves in the 

 shape of horny linear bodies, 

 toothed or serrated upon one or 

 both sides, and often combined 

 into more or less complex sys- 

 tems. If we disregard the 

 genus Dictyonema, which is best 

 referred elsewhere, the Grapto- 

 lites have an extremely definite 

 range in point of time, being 

 exclusively confined to the Silu- 

 rian deposits. They attain their 

 maximum of development in the 

 basement - beds of the Silurian 

 (Quebec group of Canada and 

 Skiddaw Slates of England), 



are abundantly represented in the higher portion of the 

 Lower Silurian, and die out altogether before the close of 

 the Upper Silurian period. 



Excluding the genera Dictyonema, Dendrograptus, Ptilograp- 

 tus, and Callograptus, the Grraptolitidce may be defined by the 

 possession of a compound polypary, consisting of a tubular 

 chitinous investment enclosing the coenosarc, giving origin 

 to numerous cup - like " cellules " or " hydrothecse," each 

 of which protected a polypite. The polypary was free, 

 and was not attached to any foreign body ; and the poly- 

 pites were not separated from the coenosarc by any par- 

 tition. Lastly, the polypary was almost always strength- 

 ened by a chitinous rod or fibre, which is termed the 

 " solid axis," and which is analogous to the chitinous rod 



Fig. 53. Oldhamia antiqim, natural 

 size (after Salter). Cambrian. 



