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XII. — On the Occurrence of Fossil Xanthidia and Polythalamia in 

 Chalk. By Henry Deane, Esq. 



(Read October 15, 1845). 



The occurrence of Xanthidia in either a recent or fossil state, has 

 not hitherto been observed in any other situation than in flint nodules 

 of the upper chalk formation, and it has been a matter of surprise that 

 the chalk itself surrounding the flints should not have afforded 

 evidence of their existence. So perfectly free from them does it 

 appear, that the prevailing opinion has been, they had not an existence 

 out of the flints themselves, that is to say, floating loosely as indepen- 

 dent beings in the seas of the cretaceous period ; but that they were 

 immediately connected with the substance, whether animal, vegetable, 

 or spongeous, which, in the course of decomposition, has been replaced 

 by the silica, now forming the nodules in which they are found. My 

 own observations had led me to the conclusion that they were inde- 

 pendent animal existences, — whether of sponges or not I could not 

 tell, — and in all probability to be found elsewhere than in flints. 

 These views have lately been remarkably confirmed by my finding 

 them abundantly in a piece of chalk, picked up in the course of a 

 ramble along the beach between Folkstone and Dover. Proceeding 

 along the beach from Folkstone towards Dover, and at a short distance 

 before arriving at Lydden Spout and the ruins of Round Down Cliff, 

 a grayish kind of chalk, having no flint nodules, so far as I could find, 

 but containing quantities of nodules of iron pyrites, juts into the sea, 

 and forms the beach for some distance. Many of these pyrites had 

 the shape of shells which had formed their nuclei, and one piece con- 

 tained something like charred wood. The appearance of fossils so 

 remarkably different from those in the bed of chalk immediately 

 above them, led me to conclude that there might be other organic re- 

 mains of an interesting nature, for microscopic examination. I there- 

 fore cut out a piece of pyrites of singular form, with the adherent chalk, 

 and afterwards dissolved it out by means of hydrochloric acid. The 

 copious insoluble sediment left after the action of the acid, being ex- 

 amined by the microscope, exposed to view bodies similar to, if not 

 identical with, the Xanthidia in flints ; and I could clearly recognize X. 

 spinosum, ramosum, tubiferum simplex, tubiferum recurvum, mal- 



TRANS. MIC. S0C. VOL. II. G 



