HYDROIDA. 



205 



slender, transverse processes or dissepiments, and they bear small 

 horny cups or " hydrothecae " (" cellules ") like those of the Grapto- 

 lites. 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 Poly zoo) ; but the latter 

 have a calcareous skeleton, and have no " hydrothecae." Besides the 

 above-mentioned genera, Callograptus and Ptilograptus (fig. 87) may 

 with great probability be referred to the Thecaphora ; as may, per- 

 haps, be the obscure fossils Buthograptus and Thamnograptus. All 

 these genera are found in the Ordovician or Silurian rocks. 



Oldhamia. — The singular structures described under the genus 

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

 Hydivzoa ; though their true nature is altogether uncertain. Old- 

 hamia occurs in certain green and purple grits of Lower Cambrian 

 age, at Bray Head, in Wicklow, Ireland, where the supposed fronds 

 are found in great abundance, matted together, and spreading over 

 the surfaces of the strata. A form of Oldhamia is also said to occur 

 plentifully in the Potsdam Sandstone 

 (Upper Cambrian) of Wisconsin, in 

 North America, and still another form 

 has been described by Barrois from 

 the Cambrian rocks of the Pyrenees. 

 Oldhamia antiqua, the commonest 

 form, consists of a central thread-like 

 axis from which spring bundles or 

 umbels of short radiating branches 

 (fig. 88), at regular intervals ; whereas 

 in the so-called Oldhamia radiata the 

 branches radiate from a central point 

 in all directions. Oldhamia has been 

 variously referred to the Sertularian 

 Zoophytes, to the Polyzoa, and to the 

 Algce. It has not, however, been as 

 yet satisfactorily shown that Oldhamia 

 is truly organic, and it is quite pos- 

 sible that the structure so called is the 



result of purely inorganic agencies. If really organic, it would seem 

 more than probable that Oldhamia is referable to the vegetable king- 

 dom, and that it belongs to the Algce. This is the view held by 

 Barrois, who compares Oldhamia with some of the types of Algce 

 which are placed in the family of the DasycladecE. 



Order 3. Trachymedus^e. — The Trachymedusce constitute a 

 peculiar group of the " Jelly-fishes," which agree with all the organ- 

 isms grouped under this popular designation in the possession of a 

 gelatinous swimming-bell, from the under side of which is suspended 

 a single " polypite." From the margin of the swimming-bell depend 



Fig. 88. — Oldhamia antiqua, natural 

 size (after Salter). Cambrian. 



