FOSSIL HYDROZOA. 161 



Graptolites. Dictyonema ranges from the Upper Cambrian 

 to the Middle Devonian. The genus bears a close superficial 

 resemblance to the Fenestellce or Lace-corals (belonging to the 

 Polyzoa) ; but the latter have a calcareous skeleton, and have 

 no "cellules." Besides the above-mentioned genera, Callo- 

 graptus and Ptilograptus may with great probability be 

 referred to the Sertularida ; as may, perhaps, be the obscure 



Fig. 52. Dictyonema retiforme, Hall. (After Hall.) 



fossils Buthograptus and Tliamnograptus. All these genera 

 are Silurian in age. 



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 un- 

 certain. Oldhamia occurs in certain green and purple grits 

 of Lower Cambrian age, at Bray Head, in Wicklow, Ireland, 

 where the fronds are found in great abundance, matted 

 together, and spreading over the surfaces of the strata. A 

 species of Oldhamia is also said to occur plentifully in the 

 Potsdam Sandstone (Upper Cambrian) of Wisconsin, in North 

 America. Oldhamia antiqua, the commonest species, consists 

 of a central thread-like axis from which spring bundles or 

 umbels of short radiating branches (fig. 53), at regular in- 



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