TRACKS OF INVERTEBRATE ANIMALS. 27 



des Algues Fossiles/ having- the same object as the pre- 

 ceding one"^. These two volumes embody every argument 

 that can be advanced in favour o£ the vegetable origin o£ 

 the objects in dispute. Much of the discussion turns 

 upon the point illustrated by figs, i & 3 of my present 

 memoir^ viz. that nearly all the debated structures stand 

 out in prominent demi-relief from the undersides of the 

 slabs of which they form a part; and that^ as is conspi- 

 cuously the case with my specimens_, what ought to repre- 

 sent the substance of the supposed organism is merely an 

 extension of the inorganic rock overlying the sculptured 

 surfaces. M. Saporta takes much pains to show that many 

 unquestionable fossil plants are found in this same condition 

 of demi-relief. This is true ; but we find abundance of the 

 same plants in difi'erent conditions, in which substance and 

 even structures are equally preserved. Hence we are able 

 to identify the specimens seen only in semi-relief by the 

 aid of the more perfect examples. But in the case of 

 such specimens as my figs, i & 3, we have hitherto failed 

 to obtain any trace of either substance or structure. M. 

 Saporta, in his latest memoir_, seems to have found some 

 specimens of the genus Biserites, in which he can trace 

 what he describes as " le contour entier de la Bilobite.''' 

 This only shows a possibility that one of the many objects 

 to which the name of Bilobites has been given may have 

 been plants. 



These views were attacked in a formidable manner by 

 Dr. Nathorst in the memoir above referred to. This 

 important memoir embodies the results of a series of exact 

 experiments, in which various aquatic animals were allowed 

 to travel under water, leaving behind them very definite 

 tracks in fine mud as they did so. Dr. Nathorst succeeded 

 in obtaining very perfect casts of those tracks, and, in 



* The resemblance of M. Saporta's figure of Vexilliim Bcsglandi, on p. 42 of 

 the latter voUime, to my fig. 15 on Plate IIT. is too striking to be overlooked. 



