77 



XII. — On Fossil Xanthidia. By Henry Hopley White, Esq. 

 Read February 16, 1842. 



I have, for some time past, directed my observations to that inte- 

 resting portion of fossil Infusoria called Xanthidia, and which, in the 

 recent state, forms a genus of the tenth family of the class of Polygas- 

 trica, called Bacillaria. In this genus the animalcule is unattached, 

 and free from any pedicle or stalk ; having a globular form, generally 

 spherical, but in some of the species occasionally of an irregular oval. 

 All the species of this genus have the lorica or external covering se- 

 mi-transparent, and invested with tentacula, varying in form, number 

 and dimensions : from these tentacula, each of the different species 

 may, with little difficulty, be discovered. The arrangement which 

 I am about to submit to the Society, of the different species of this 

 interesting group of fossil Infusoria, is the result of much careful 

 examination of some thousands of specimens, in flint collected from 

 various chalk districts of England, and in some gravel flints. 



The name Xanthidium* is derived from the Greek word HAN0O2, 

 yellow, the prevailing hue of these minutissimal forms of animal exis- 

 tence. 



All the other Xanthidia I have met with (comprising many thou- 

 sands of individuals) can be distinctly, and, as I think, satisfactorily, 

 arranged under eleven species. There are individuals occasionally 



* When the above paper was read before the Society, I comprehended under the 

 genus Xanthidium a species which I called globosum, and placed it as the first species. 

 I so named it because it had merely a globular form, without any tentacula or spines 

 that I could discover. I think it is quite distinct from the fossil genus Volvox, which 

 is rarely to be met with, and which is, I believe, furnished with an aperture, and trans- 

 parent, having the appearance of a spherical sac or skin, none of which characteristics 

 belong to the animalcule I called Xanthidium globosum, that being nearly opaque, 

 but slightly semi-transparent towards the verge of its circumference, and without any 

 apparent aperture. It is generally found among the various species of Xanthidia, and 

 like them orbicular in form, and of a yellowish colour; and is the only other fossil ani- 

 malcule which I can discover, besides the eleven species enumerated and described in 

 the above paper, which has a well-defined spherical lorica or covering: it varies in di- 

 mensions from the ^ to the J, of an inch. Upon further consideration, and in con- 

 formity with the suggestions of the Council, I have rejected this animalcule as forming 

 a species of the genus Xanthidium, because it is without any tentacula or spines, and, 

 therefore, not coming within the genus Xanthidium as defined by Ehrenberg. It is 

 figured as 12 in the plate, and may be called Globosum, forming a genus of animal- 

 cules distinct from Xanthidium. 



