THE society's NOTE-BOOKS. 41 



DESCRIPTION OF PLATE IL, Upper Portion. 



Figs. 1 and 3. — Li the upper half the slides are represented unpolar- 

 ised ; in the lower half the colours induced by polarisation are 

 given ; t. c. t. c, Fig. 1, a (supposed) twin Crystal. 



Fig. 2. — Portion of the accessor}^ mineral spoken of as having a 

 greenish colour ; in the upper half of the circle, the natural 

 appearance is represented ; in the lower half, the changes 

 induced (after the removal of the upper prism, as directed), by 

 rotating the sub-stage prism. 



Xanthidia in Flint (PI. 3, lower portion). — These curious 

 and obscure organisms, when first observed, made such a sensation 

 that, it is said, the quantity of flint nodules broken up in the 

 search for specimens amounted altogether to many tons. They 

 are often found plentifully in the fossil state, as many as 20 

 having been detected in a piece of flint i-i2th inch in diameter ; 

 and Hogg says — " It is rare to find a gun-flint without them." 

 There has been much discussion as to what these bodies really 

 are. At first, Ehrenberg and others thought them to be Infusoria, 

 but more recently they have been described as the fossil Sporangia 

 of Desmidiaceae ; their skeletons being shown to be composed of 

 a horny substance, and not of silica, as was once supposed. In 

 form they vary much ; generally, they are small, flattened spheres, 

 either smooth, bristly, or furnished with spines, some of which 

 are simple, others branched at the extremities. In one species, 

 the tip of each spine is expanded like a sucking-disc. Some- 

 times a membrane may be traced, either covering the spines or 

 entangled with them. Some specimens exhibit denf spines, and 

 torn margins— appearances which forbid the idea that they were 

 originally siliceous in structure. 



Similar bodies have been found in the Chalk near Dover, as 

 well as in flints ; and recent specimens have been obtained from 

 the Thames mud, near Greenhithe. But all these deposits are 

 believed to be marine, whereas the Desmids, as far as we know, 

 are confined to fresh water : how, then, has the connection 

 arisen between them? 



J. H. Green. 



I well remember the period referred to by Mr. Green, when 

 stone-breaking was the rage, and the many pleasant hours I 

 enjoyed in my search for these singular forms. 



My practice was to split a flint, and then with a light 

 hammer chip ofl" small and very thin flakes, which I placed in 

 Balsam, and by this means could select such as possessed a 

 desired figure. This is not possible in a polished slide. Some 

 time ago I circulated a slide containing 125 Xanthidia. 



A. Nicholson, 



