M. Prevost on a New Gyrogcmite, 61 



charae form part of one of the newest deposits, the formation of 

 which, if it does not belong to the present period, is at least more 

 recent than the diluvian deposits on which the Forfar marls rest. 

 On the contrary, the gyrogonites observed by M. Constant Pre- 

 vost, in the upper fresh water limestones of the neighbourhood 

 of Paris, are connected with an older formation, probably ante- 

 rior to the great revolution which has left the globe in its pre- 

 sent state. This difference of position it is of importance to 

 remark, because it serves to connect, by insensible shades, the 

 productions of nature as it exists at present, with those of the 

 period when the soil on which we now live was formed. 



The new gyrogonite observed by M. Constant Prevost, is not 

 less abundant in certain localities of the heights of Montmorency, 

 than the gyrogcynites medicaginula along with which it occurs. 

 It differs essentially from this latter in its form, which is elon- 

 gated ovoidal ; in its size, which is less by a half or even two- 

 thirds, a circumstance which renders it scarcely perceptible with- 

 out the aid of a glass ; lastly, in the number of spiral turns, 

 which, in place of being six, vary from nine to ten. These 

 characters prevent our confounding the new gyrogonite with the 

 two species described by M. Ad. Brongniart, and they serve to 

 approximate it to the fossil which M. Ch. Leyell has made 

 known, and consequently to the capsules of Clmra vulgaris at 

 present so common in the waters of numerous marshes, which 

 exist upon the very soil filled with analogous fossil bodies. The 

 distinction becomes so much the more difficult to be established, 

 that in the seeds of the same species of chara^ gathered from 

 the same stalk, there are perceived in the size, the more or less 

 elongated general form, the number of spiral turns, differences 

 which some might consider sufficient to estabhsh several genera. 

 M. Constant Prevost has found similar variations in the fossil 

 gyrogonites, of which he has a great number in his possession. 

 Most of these, as in the G. medicaginula^ have lost their outer 

 covering, and the part preserved, or rather replaced, is only the 

 internal nucleus of the capsule, of which, however, the impres- 

 sion is frequently preserved in hollows in the compact silex. 

 These petrifactions are seen principally in blocks of white and 

 compact fresh water silex, which affect irregular rounded forms, 

 representing geodcs, which are disseminated without order in 



