E. W. Wether ell — Xanthidia in the London Clay. 29 



of the means of isolation and of the appearance of these minute 

 fossils. 



I may here mention that all writers ^ previous to Mr. Henry Deane 

 speak of them as spherical bodies; but these earlier observers, including 

 the discoverer Ebrenberg, had only examined fossil Xanthidia in 

 chips of flmt; whereas Mr. Deane ^ succeeded in isolating them. Mr. 

 Deane took a fragment of Grey Chalk, obtained from the beach between 

 Folkestone and Dover, and dissolved it in hydrochloric acid, and in 

 the residue he discovei'ed these organisms — which he describes as 

 not being true spheres, but somewhat flattened, having a remark- 

 able resemblance to some gemmules of sponges, and having a 

 circular opening in one of the flattened sides. Mr. Deane adds that 

 on submitting some individuals to pressure between glass plates, 

 they were torn asunder laterally like a horny substance, and the 

 spines in contact with the glass were bent, while some specimens after 

 soaking in water became flaccid showing that they were not siliceous. 



Having thus briefly described Mr. Deane's results, I will proceed 

 to the description of the London Clay forms and the method of 

 obtaining them. 



My attention was first called to them in the finer siftings of the 

 washing of clay obtained from the cliff near Minster, Sheppey. 

 This clay I obtained at a height of about four feet above the beach, 

 and it is of a greenish colour — all the clay above and below being 

 brown and much more compact. A great number of organisms can 

 be seen in this clay without the aid of a magnifying-glass, whereas 

 the brown clay contains few fossils. 



The first specimens of Xanthidia found were very much damaged, 

 so I altered the method of washing in the following way. The clay 

 was disintegrated on a piece of perforated zinc immersed in water, 

 and after an hour or so the zinc was gently shaken and all the finer 

 particles went through, leaving the larger fossils, such as Gasteropods, 

 vertebrae, and teeth of fishes, etc., on the zinc, which was then 

 removed, and the finer material was again sifted in the wet state 

 through a wire sieve 90 x 90 holes to the square inch ; this separated 

 out the larger Foraminifera (which exist in this deposit in great 

 numbers of tlie genera Cristellaria, Marginulina, Nodosaria, etc.), and 

 the finest material was then washed and dried in the ordinary manner, 

 and gave far better results than the material resulting from washing 

 in the ordinary way. A microscopical examination of this finest 

 sifting showed a great number of Xanthidia, which for the most part 

 follow two types which can be easily distinguished, although there 

 are a great many minor varieties of each of them. In many cases 

 these organisms are found joined together, in pairs or with five or 

 six individuals massed together. 



Their characters are as follows : shape, lenticular ; some specimens 



1 Ehrenberg, 1836, Bericht. des Akademie Wissenscliaften zu Berlin, p. 114. — 

 vide also Abhandlungen Konigl. Akad. Wissenschaften zu Berlin, 1836 (published 

 1838), p. 114.— Turpm, Comptes Eendus de I'Academie des Sciences, 1837. 



- Micros. Soc. Trans. Oct. 15th, 1845, vol. ii. p. 77. 



