Sketch of the Infusoria of the family Bacillaria. 291 



There has been much discussion as to the real nature of the 

 bodies in flint which so closely resemble the recent Xanthidia ; 

 Turpin mistook them for eggs of Cristatella, to which however 

 they have but little resemblance. Ehrenberg gives figures in his 

 small treatise, " die Fossilen Infnsorien und die lebendige Dam- 

 merde," PI. 1, figs. 2, 12 to 17, which show the closest resem- 

 blance between the recent and the fossil species. It is remarka- 

 ble however that the recent species are inhabitants of fresh water, 

 while flint is undoubtedly of marine origin. I have not seen dis- 

 tinct motion in any of our species. 



1. Xanthidiumf ramosum. Corpuscles globular, separate or binaiy, 

 spines scattered, terminated in three or more points -g\ to ^V 'inc. Fos- 

 sil in gun flints. 



2. Xanfhidium . (Fig. 15, PI. 1.) Binaiy, each portion 



having numerous rather long arms terminating in three diverging points. 



This very beautiful American species is, when living, of a fine 

 green color. Its carapace is hard and apparently siliceous, as it 

 retains its form in spite of the action of fire and acids. Every 

 living specimen of this species which I have seen, has been com- 

 posed of two symmetrical portions, as shown in the figure. These 

 bodies when in the position shown in the figure, have much re- 

 semblance to Ehrenberg's drawings illustrating the spontaneous 

 division of some of the species. When thrown into other posi- 

 tions, so that the line of union of the two portions is not seen, it 

 may easily be mistaken for a simple spherical body bristling with 

 arms. The two portions often separate after death, and may 

 then be mistaken for individuals resulting from spontaneous di- 

 vision, but by throwing them into various positions by means of 

 a compressor, the orifice corresponding to the line of separation 

 of the two original parts may always be seen. The same remark 

 will apply to the next species also. 



It occurs not unfrequently in a subalpine pond a few miles 

 from West Point ; it is also occasionally found in ditches, in peat 

 bogs, &c. 



3. Xanthidium . (Fig. 16, a, &, PI. 1.) Binaiy, each por- 

 tion somewhat triangular and terminating at each angle in three short, 

 diverging arms, each having three small diverging points. 



The smaller size and triangular form of these bodies, make me 

 think them of a different species from the preceding. I have not 

 seen any intermediate forms, although they occur together in 

 about equal abundance at the above mentioned locality. 



