384 Correspondence. 



Cavallo. The lava issuing copiously from this enormous fissure 

 flowed through the Fosso de la Vetrana, between the extremity of 

 Monte Somma and the Observatory, into the cultivated country, and 

 partly destroyed the villages of Massa di Somma and San Sebastiano. 

 The author had succeeded in photographing in a most admirable 

 manner various parts of the mountain after the eruption, includ- 

 ing the crater itself ; and the large series of views so taken were 

 exhibited at the Meeting. 



C O laiaiHl S !=• O n^IDE liTG S . 



ON BLOCKY EOCK SUEFACES. 



SiR^ — While very much interested in Mr. Poulett Scrope's article in 

 your last Number " On Blocky Eock Surfaces," I feel bound to state 

 that the explanation there given of the Mock]! structure cannot apply 

 to Scawfell, since the mountain is made up of bedded and altered ash, 

 and certainly is not in any sense "the upper portion of a protruded 

 mass which reached its present position in a state of igneo-aqueous 

 liquefaction." 



Keswick, Jtdy 9th. J. Clifton Wakd. 



PALEOZOIC ECHINODEEMS WITH OVEELAPPING PLATES. 



SiK, — In the July Number of the GBOLoaiCAL Magazine, Mr. J. 

 Young pointed out the resemblance between the plates of the Car- 

 boniferous genus ArcliCBocidaris, McCoy, and the new Calveria hystrix, 

 W. Thomson. Mr. Young further made some interesting remarks 

 on the Carboniferous fossil. 



In addition to ArchcBocidaris, another Palceozoic genus of Echino- 

 dermata possesses imbricating plates in its test. I refer to the genus 

 Lejndechinus, Hall. In this the ambulacral plates imbricate from 

 below upwards, those of the inter-ambulacral area from above down- 

 wards (Hall, Descr. New Sp. Crinoidea, Prelim. Notice. Albany, 

 p. 18). Prof. Hall placed LepidecMnus as a subgenus of Archceo- 

 cidaris, a reference which Mr. Young's observations woi;ld go some 

 way to bear out. On the other hand, Messrs. Meek and Worthen 

 have observed that only the marginal inter-ambulacral plates of the 

 lower side of the test of LepidecMnus carry primary tubercles 

 (Pal. 111., vol, 2, p. 295), as do the same plates in Perischodomus, 

 McCoy. Could it be shown that the latter also had imbricating 

 plates, there would be grounds for the supposition that the two 

 genera were indeed very closely related. LepidecMnus occurs in the 

 Burlington Group (Carboniferous). 



Edinburgh, July 9th, 1873. E- Ethekidge, JuN. 



