30 E. W. WethereU — Xanthidia in the London Clay. 



far flatter than others, but this may be due to pressure ; in some the 

 spines (tentacula of White)' appear to be round the edge, and also 

 springing out obliquely from the flattened sides, and in others they 

 appear to be round the edge only. The flattened surface of the 

 organism is very variable in shape, sometimes almost truly circular, 

 in other cases elongated, and often very irregular in contour. 



The length, thickness, and number of these spines varies very 

 considerably, giving rise to the two types above referred to. One 

 type has the central body-portion circular and small with a large 

 number of regular radial spines, almost as long as the diameter of 

 the body portion, and very thin, the ends much branched. The 

 other type has a much larger body, more irregularly shaped, fewer, 

 shorter and thicker spines, which branch out very irregularly, 

 crowded together in one part and scanty in other parts. 



When viewed by reflected light, Xanthidia appear to be white, 

 or pale brownish, and the spines are transparent. If seen by 

 transmitted light, however, when mounted in glycerine, the body 

 portion becomes more or less translucent, and is of a distinct green 

 colour. This greenish body contains black spots, and in some few 

 cases the whole body, except the very edge, is opaque. Glycerine 

 shows the whole form better than any other medium, Canada 

 Balsam having an index of refraction which prevents the spines 

 being seen. These spines can be bent by pressure when wet. 



Saffronine stain was taken up by the body, but the spines were 

 but little affected. 



The diameter of these minute organisms is about -i-j-nmi., coin- 

 ciding exactly with the measurement of the Chalk forms. 



With reference to the greenish clay which contains Xanthidia, 

 I should mention that it is not a distinct bed or band, but a patch, 

 and contains great quantities of fish-remains (vertebrae, spinous pro- 

 cesses, otoliths, scales, etc.), besides the Mollusca and Foraminifera. 

 Although a patch, it occurs in a band of somewhat laminated clay, 

 which I have traced for several hundred yards along the clifls, 

 in which occur other fossiliferous patches, which do not contain 

 Xanthidia, as far as I have been able to see, although I have 

 examined several samples. The Xanthidia patch, as I found it, 

 was about a yard long, some three or four inches thick, and 

 extended about eighteen inches or possibly farther, into the cliflf; 

 but near that point the clay appeared less fossiliferous to the 

 naked eye, and was probably at the end of the patch. 



I wish to mention that, through the kindness of Prof. Judd, the 

 work in connexion with this paper was performed in the geological 

 laboratory of the Koyal College of Science, South Kensington. 



1 H. H.White, Trans. Micros. Soc. vol. i. p. 77, on Fossil Xanthidia, Feb. 1842, 



