XANTHIDIA. 441 



which gradually enlarge from the centre to the margin ; 

 the angles of the cross are filled up with hexagonal cella 

 as previously noticed. (Fig. 224, No. 4.) 



The unskilled manipulator may for some time endeavour 

 to adjust a slide, having a piece of glass exposed not larger 

 in size than a pea, on which he is informed an invisible 

 object worthy his attention is fixed, before he is rewarded 

 by a sight of Triceratium farus, extracted from the mud of 

 the too-muddy Thames. The hexagonal markings, cells, 

 are beautiful, and at each corner there is a curved pro- 

 jecting horn or foot. (Fig. 225, No. 10.) In Bermuda earth 

 there is a small species found, which has its three margins 

 curved ; and also a curious species, which resembles the 

 triradiate spiculum of a sponge. 



It is remarkable how, in these minute and obscure 

 organisms, we find ourselves met by the same difficulties 

 concerning any positive laws governing the formation 

 of generic types, as in larger and more complex forms of 

 animal and vegetable life. It appears as if we could 

 carry our real knowledge little beyond that of species; and 

 when we attempt to defiue kinds and groups, we are en- 

 countered on every side by forms, which set at nought our 

 definitions. 



Man even uses infusorial remains as food ; for the berg- 

 mehl, or mountain-meal found in Swedish Lapland, and 

 which, in periods of scarcity, the poor are driven to mix 

 with their flour, is principally composed of the flinty shells 

 of the Gallionella sulcata, Navicula viridis, and Gompho- 

 nema geminatitm Dr. Trail, on analysing it, found it 

 to consist of 22 per cent of organic matter, 72 of silica, 

 5-85 of alumina, and - 15 of oxide of iron. This would 

 seem to be the same substance described by M. Laribe the 

 missionary, and put to a similar use in China : " This 

 earth," he says, " is only used in seasons of extreme 

 dearth." 



Xanthidia. — In conjunction with the skeletons of the 

 former species it will be as well to offer a few remarks upon 

 animals long classed with Infusoria, and but rarely found 

 except in the fossil state. There is every reason to believe 

 that the Xanthidia, double-bar animalcules, are sporangia 

 of Desmidiactce. In proof of this it can be shown that 



