442 THE MICROSCOPE. 



their skeletons are composed of a horny substance, and not 

 of silica, as was once supposed. 



The name Xanthidia is derived from a Greek word sig- 

 nifying yellow, that being their prevailing hue. They are 

 found plenteously in a fossil state, imbedded in flint, as 

 many as twenty being detected in a piece the twelfth of 

 an inch in diameter ; in fact, it is rare to find a gun-flint 

 without them. When living they may be described as 

 having a round transparent shell, from which proceed 

 spikes varying in size and shape. One kind, found by Dr. 

 Bailey in the United States, was of an oval form, the 288th 

 of an inch in length; and another species circular, found 

 by the late Dr. Mantell, at Clapham : both were of a 

 beautiful green colour. Specimens of Branched Xantlii- 

 dium, found in flint by Dr. Mantell, were from the 300th 

 to the 500th of an inch in diameter. Mr. Ralfs says: 

 " That the orbicular spinous bodies so frequent in flint, 

 are fossil sporangia of Desmidiacece, cannot, I think, be 

 doubted, when they are compared with figures of the 

 more recent forms. Indeed, the late Dr. G. Mantel], who, 

 in his Medals of Creation, without any misgiving, had 

 adopted Ehrenberg's ideas concerning them, changed his 

 opinion ; and in his last work regards them as having been 

 reproductive bodies, although he is still uncertain whether 

 they are of vegetable origin." 



The fossil forms vary as much as recent Sporangia, in 

 being smooth, bristly, or furnished with spines, some 

 are simple, and others branched at the extremity. Some- 

 times, a membrane may be traced, even more distinctly 

 than in recent specimens, either covering the spines, of 

 entangled with them. Writers have described the fossil 

 forms as having been siliceous in the living state ; but 

 Mr. Williamson informs us that he possesses specimens 

 which exhibit bent spines and torn margins ; and this 

 wholly contradicts the idea that they were siliceous be- 

 fore they were embedded in the fiint. In the present 

 state of our knowledge, it would be somewhat premature 

 to identify the fossil with recent species ; it is better, 

 therefore, at least for the present, to retain the names 

 bestowed on the former by those observers who have do- 

 scribed them. 



