136 Correspondence. 



Dumfriessliire, and their contained fossils), who both examined my 

 specimens, and himself collected a number, in a visit which he paid 

 with me to Moffat last summer. 



Leaving the subject of the ovarian capsules, Mr. Carruthers appears 

 to think that the Diplograpsus Whitfieldii of Hall is identical with 

 the D. tricornis described by himself. This, however, is certainly 

 not the case, the two being distinguished, amongst other differences, 

 by the obvious character that the former is provided with but a 

 single mucronate radicle, whilst the latter is furnished with three. 

 I have found D. Whitfieldii at Glenkiln Bum, in Dumfriesshire, but 

 I am not aware of its occurrence having been noticed elsewhere. 



It seems to me that Mr. Carruthers is likewise wrong in the asser- 

 tion, that Diplograpsus pristis, Hisinger, is provided with spinose or 

 mucronate cellules. I should speak more positively on this point, 

 but I am not able to refer to the original figures by Hisinger, and 

 can only judge from the various figures in Hall, and from McCoy's 

 description. Certainly I have myself never seen a single specimen 

 in which this was the case, and I should be inclined to suggest (not 

 having seen the specimens upon which Mr. Carruthers has founded 

 his statement), that he has probably mistaken for D. pristis, speci- 

 mens of the Diplograpsus quadri-mucronatus of Hall. I have found 

 this beautiful species not uncommonly in the Moffat shales, and 

 when compressed in certain directions, it presents but a single row 

 of spines, thus coming closely to resemble the ordinary form of D. 

 pristis, and differing chiefly in the mucronate cellules. 



I am, Sir, Yours, etc., 



Henry Alletne Nicholson. 

 Edinburgh, February 6tk, 1 867. 



MR. MAW, PROFESSOE JUKES, AND OTHERS ON DENUDATION. 

 To the Editor of the Geological Magazine. 



Deab Sik, — Since the appearance of my articles on the origin of 

 Escarpments and Valleys (Geol. Mag. April and July 1865 ; Feb. 

 and Sept. 1866), you have given insertion to an array of contribu- 

 tions more or less in favour of subaerial denudation. As I find a 

 full reply would not come within any reasonable compass, in your 

 Magazine, and as several observations have been made which render 

 silence on my part no longer desirable, would you kindly find room 

 for a few brief remarks. 



Planes of Marine Denudation. — As on this point I have been mis- 

 understood, permit me to remark that in asserting that the sea is not 

 a levelling agent, I, of course, meant that the sea only planes down 

 its bed to an extent proportionate to the amount of flat surface 

 presented by the land at any given time. This planing down 

 process is far from universal. It is nearly absent in Archipelagos, 

 and on continuous coast-lines it requires a very slow, uniform, and 

 nnintermittent rise or fall of the land. Table-lands, with surround- 

 ing declivities or escarpments, are planes of marine denudation, and 

 so are plains surrounded by acclivities or escarpments, unless a mere 



