630 On fossil impressions in (he Ponar valley. [Dec. 



the sam3 age, whether these be slate or limestone. On the other hand, 

 we know that mere delineations on the surface of particular rocks, differ 

 with the constitution of the rock in which they occur, are uncertain as 

 to size, and are without any fixed regularity in the proportion of the 

 different parts to each other ; proving them to be either the result of 

 mechanical increment or of chemical attraction. Respecting organic 

 fossils, Cronstedt says, " They are distinguished by an organic struc- 

 ture more or less imperfect, of which as long as they bear any marks 

 we are to reckon them as fossils of a foreign* species." With respect 

 to your remarks on the rings, I can only account for the part of the 

 lower and upper portions being both visible, by supposing the bodies 

 to which they belonged to have been soft enough to yield to lateral 

 pressure, and to have been thus converted into superficial substances. 

 Others again, as fig. 1, may have been exposed to compression, which 

 acted longitudinally, so as to destroy their length, but preserve the 

 lateral dimensions. 



It is unnecessary to remark that this explanation would not apply to 

 any univalve shell with a regular spire ; and that of univalve shells 

 without regular spires, Dentalium is the only genus to which these ap- 

 pearances can be referred. The generic character of Dentalium is 

 " shell awl-shaped, open at both ends." The rings are sufficiently 

 characteristic to distinguish the species ; but until we can procure good 

 specimens, it is premature to be positive as to the place these fossils 

 should occupy. I know the danger of touching fossil drawings with- 

 out the specimens before one's eyes, and what shakes my confidence 

 in the drawing attached to the former notice now is, that though it 

 was accurately sketched from the specimens, yet it was finished from 

 recollection only. 



With respect to the drawing here attached, it is calculated to mis- 

 lead as to the true nature of the fossils ; were the figures complete, 

 they would be found to be awl-shaped, the ends nearly equal in size, 



to fig. 3, except that the Ponar fossil appears to have been perforated in the 

 centre, while Mr. B.'s figure is merely grooved by external striae, but in this re- 

 spect, Mr. B. remarks, there is great variety — may not the Ponar fossil be a Be- 

 lemnite, so worn and changed by the lapse of ages, as only to present the marks of 

 former cells : the outer crusts being destroyed, and the traces of septse and si- 

 phuncle only remaining — but taking the aggregated form of the rings, and assum- 

 ing them to have been a shell ; it certainly would have agreed with the moderngenus 

 Dentalium ; but if by that we imply also the nature of the animal which formerly 

 occupied it, we then go too far in attempting to define so imperfect a trace of 

 the organization of a former world. In a chronological arrangement this fossil 

 must take its place amongst the remains of the earliest created beings. 



* " Foreign species," as here used, means foreign substance. 



