THE 



GEOLO&ICAL MAGAZINE. 



No. XCVIII.— AUGUST, 1872. 



I. — Notice of a Fossil Hydractinia from the Coralline Crag. 

 By Prof. George James Allman, M.D., F.R.S., etc. 



AMONG the very few known instances of fossil hydroids, the 

 genus Hydi-actinia must be inchided. Under the name of 

 Cellepora ecJiinata, M. Michelin has described a fossil from the 

 Subapennine group of Asti, and from the Superior Fallunian of 

 Bordeaux and Dax (Michelin, Icon. Zooph., p. 74, pi. xv., fig. 4.). 

 M. Fischer has drawn attention to the fact, that the Cellepora 

 echinata of Michelin is really a Hydractinia encrusting a Murex or a 

 Nassa, while he has himself added another fossil Hydractinia from 

 the Upper Greensand of Mans (Fischer, Bull, de la Soc. Geol. de 

 France, 2°"® ser. t. xxiv., p. 689). This he found in the collection 

 of M. Ale. d'Orbigny, where it encrusted numerous specimens of 

 Natica tubercidata, d'Orbig., from that formation. M. Fischer has 

 assigned to this species the name of Hydractina cretacea, while to 

 Michelin's species he has given that of Hydractina Michelini. 



To the two examples thus noticed by Michelin and Fischer, I am 

 enabled to add a third from the Coralline Crag of Suffolk. It occurs 

 among some Coralline Crag fossils in the collection of the British 

 Museum. It was found encrusting two specimens of Purpura 

 lapillus, one from Orford, and the other from Gedgrave, in Suffolk. 

 It covers with a continuous crust the shell over which it spreads, 

 and has a minutely alveolar structure, with its surface thickly set 

 with short blunt spines. The original chitine of the common basal 

 expansion is entirely replaced by carbonate of lime. There cannot 

 be the slightest doubt of this fossil being a true Hydractinia, and 

 indeed it is impossible to find any characters which can separate 

 it from the living Hydractinia echinata. From the mere fossilized 

 basis, however, which is, of course, all that has come down to us, 

 we should not be justified in asserting its identity with the living 

 hydroid. 



M. Fischer gives no description by which his own hydroid may 

 be specifically distinguished from that of M. Michelin, though be 

 regards the two as specifically distinct, and assigns to them distin- 

 guishing names. Indeed it is highly probable that no zoological 

 characters of diagnostic value can be detected sufficient to distin- 

 guish them from one another, or from the Coralline Crag fossil here 

 noticed, or even from our living Hydractinia echinata ; and the only 

 tangible distinction between the four hydroids is thus a purely 



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