190 Correspondence — Mr. A. C. Seward. 



Dr. Wheelton Hind believes that the " geuus " Natadites contains 

 three distinct genera, for one of which the name must be retained. 

 He proposes to retain the name for the forms called Anthracomya, 

 affirming as this word does an altogether wrong affinity for the 

 genus. (The name Naiadites was proposed in 1860 ; Anthracomya 

 in 1861.) 



Dr. Hind is not able to state that any of the species submitted to 

 him by Sir J. W. Dawson are the same as British foi-ms. The shell 

 originally described as Naiadites carhonaria is, he has no doubt, an 

 Anthracoptera. He gives notes on N. arenaria, N. angulata, and 

 N. Icevis. 



PRESERVATION OF FOSSIL PLANTS. 



Sir, — In answer to certain questions of a Correspondent in the 

 December Number of this Magazine, I should like to offer the 

 following suggestions : — 



(i.) Mr. Wilmore asks why the "Carbonaceous covering" of such 

 Coal-measure fossils as Calamites, etc., is confined to the outside of 

 the cast. In such a plant as Calamites the sandstone or shale cast, 

 which usually represents the genus in a fossil state, has frequently 

 a layer of carbonaceous matter moulded on its surface. The sand- 

 stone or shale is simply the hardened sand or mud which fill up the 

 hollow pith of the Calamitean stem, and the layer of coal represents 

 the carbonized remnant of the woody and cortical tissues of the 

 plant stem. The thickness of this coaly covering varies in different 

 species ; and it has been suggested that we may, in some cases, 

 calculate the original thickness of the stem tissues by multiplying 

 the thickness of the carbonaceous layer by 26. It is doubtful 

 whether such a method should be looked to as likely to afford 

 accurate results ; but no doubt the coaly residue will vary consider- 

 ably in thickness according to the diameter of the stem from which 

 it has been formed. 



In impressions of fern fronds, in which the pinnules are coated 

 with a thin film of carbon, it is occasionally possible to trace the 

 outlines of the original cells of the leaf. 



(ii.) As to whether the carbonaceous layer on the surface of a 

 cast represents the whole of the carbon of the plant tissues, it is 

 difficult to say how much has escaped in a gaseous form during 

 the gradual disorganization of the tissues; certainly the amount of 

 carbon contained in a very thin layer must usually be regarded as 

 the product of a much greater thickness of plant substance. 



(iii.) The third question is : — " Why, in comparatively soft and 

 little altered freestones, should the carbonaceous layer exhibit such 

 a baked or charred appearance " ? This baked or charred appear- 

 ance should not merely be referred to the action of heat, but is in 

 all pi'obability the result of various weathering agencies and not 

 necessarily the expression of actual charring. 



Cambridge, Feb. 12, 1894. A. C. Seward. 



