150 Correspondence — Dr. Nicholson, 



centric layers, the independent pressure of each successive course 

 is diverted in a line parallel with the circumference. To carry out 

 the analogy we have merely to suppose two such semicircular arches, 

 E F E and E D E, placed base to base in contact ; the balance of 

 resistance is completed, and we get a perfect epitome of the 

 equilibrium of gravitation in the crust of the earth. Will not this 

 satisfactorily explain the point noticed by Mr. Forbes, that the actual 

 density of the earth falls short of its calculated density, on the 

 estimate of the accumulation of superincumbent pressure ? and will 

 not the lateral pressure, analogous to that existing between the 

 voussoirs of an arch, account for the horizontal force which seems 

 to have operated in the production of Slaty Cleavage ? 



George Maw. 

 Benthall Hall, Broseley, 



Feb. 10th, 1868. 



I.— THE GRAPTOLITES OF THE SKIDDAW SERIES, ETC. 

 II.— ON THE CLASSIFICATION OF GRAPTOLITES. 



Sir, — 1. In the Geological Magazine for January (p. 32), an 

 abstract is given of my paper on the Graptolites of the Skiddaw 

 Series, read before the Geological Society, December 4th, 1867. 



As the generic characters of Dichograpsus are therein mis-stated, 

 I should be glad if you will allow me to correct the error, ^ since I 

 observe that it has been reproduced in a recent paper on Graptolites. 



The presence of a corneous cup does not form a character of the 

 genus Dichograpsus, since it is present in some species of the genus, 

 and is uniformly absent in others. It likewise occurs in some 

 Tetragrapsi, whilst it is never found in others, as T. hryonoides, Hall, 

 and T. quadri-hrachiatus, Hall. Lastly, it is occasionally found in 

 some Diplograpsi, as D. bicornis, Hall. As the remainder of the 

 definition of the genus is also incorrectly stated, I may be permitted 

 to add that Dichograpsus is sufficiently defined by "the possession 

 of a frond composed of a variable number (always more than four) 

 of simple stipes, arising from a central non-celluliferous stem or 

 funicle. The stipes are monoprionidian, and are given ofi" from the 

 funicle in a radiating manner." 



n. — As a recent paper of mine on Graptolites (Ann. and Mag. 

 Nat. Hist. Jan. 1868) has formed the subject of a somewhat lengthy 

 criticism by Mr. W. Carruthers, in the Geological Magazine for 

 February, (p. 64), I trust you will afford me space for a reply. For the 

 sake of brevity as well as clearness, I will notice such points as I 

 may think necessary, in the order in which they occur in Mr. 

 Carruthers' paper, premising that I have no intention of criticising, 

 and shall simply touch upon such points as concern me personally. 



1. Mr. Carruthers finds fault with me for " summarily " dis- 

 missing the Polyzoa, and for asserting that they " have, as a rule, a 

 more or less calcareous test, and the individuals forming the compound 

 organism are not united by any organized connecting substance." 



1 The abstract here referred to, is furnished by the Assistant Secretary of the 

 Geological Society, and is merely reproduced in the Geological Magazine. — Edit. 



