FOSSIL HYDROZOA. 79 



OLDHAMIA. The singular fossils described under the genus 

 Oldhamia may be noticed here, as they have been referred to 

 the Hydrozoa ; though their true nature is altogether uncertain. 

 Oldhamia occurs in certain green and purple grits of Lower 

 Cambrian age, at Bray Head, in Wicklow, Ireland. They 

 occur in great abundance, matted together, and spreading over 

 the surfaces of the strata. Old- 

 hamia antiqua, the commonest 

 species, consists of a central 

 thread-like axis from which 

 spring bundles or umbels of 

 short radiating branches (fig. 27), 

 at regular intervals. Each branch 

 " is formed of a series of articu- 

 lations marking the positions of 

 minute cells" (E. Forbes). Old- 

 hamia has been variously refer- 

 red to the Sertularian Zoophytes, 

 to the Polyzoa, and to the vege- 

 table kingdom. The most pro- 

 bable conjecture would refer the 

 genus to the calcareous sea- 

 weeds (Salter). 



III. SUB-CLASS GRAPTOLITID^E 

 (Graptolites). 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 systems. 

 If we disregard the genus Dictyonema, which is best referred 

 elsewhere, the Graptolites have an extremely definite range in 

 point of time, being exclusively confined to the Upper Cam- 

 brian and Silurian deposits. They attain their maximum of 

 development in the Upper Cambrian Rocks (Quebec group of 

 Canada and Skiddaw Slates of England), are abundantly re- 

 presented in the Lower Silurian, and die out altogether before 

 the close of the Upper Silurian period. 



Excluding the genera Dictyonema, Dendrograpsus, Ptilograp- 

 MS, and Callograpsus, the GraptolitidcB may be defined by the 

 possession of a compound polypary, consisting of a tubular 

 chitinous investment enclosing the ccenosarc, giving origin to 

 numerous cup-like " cellules " or " hydrothecae," each of which 

 protected a polypite. The polypary was free, and was not at- 

 tached to any foreign body ; and the polypites were not sepa- 

 rated from the coenosarc by any partition. Lastly, the poly- 



