Dr. T. Sterry Hunt — On Volcanic Action. 245 



2. PalcBcoma Colvini, Salter, op. cit. p. 328 (loc. same as foregoing sp.)- 



3. ,, cygnipes, Salter, op. cit. p. 329 ,, ,, 



4. „ {Bdellacoma) vermiformis, Salter, op. cit. p. 329 (loc. ibid.). 



5. „ (JRliopalocoma) pyrotechnica, Salter, op. cit. ditto. 



6. „ spinosa, Billings, Trenton Limestone. Montmorency Falls, Canada 



East. 



VIII. Paleodiscus ferox, Salter, 1857, op. cit. p. 333, pi. ix. fig. 6. Lower Ludlow 

 Eock, Leintwardine. 

 IX. Petraster bellulus, Troost, Niagara Group, Grimsby, Canada West. 



ii. X. 1. Frotaster Miltoni, Salter, 1857. op. cit. pi. ix. fig. 4. Lower Ludlow, Leint- 

 wardine, etc. 



2. „ leptosoma, Salter, 1857, op. cit. pi. ix. fig. 5 (locality ibid.), 



3. ,. SedgwicJcii, Forbes, 1849, Mem. Geol. Surv. Decade i. pi. iv. 



Ludlow Rocks, Underbarrow, Kendal, Westmoreland. 



4. „ Salteri, Sowerby, 1845, Quart. Journ. Geol. Soc, vol. i. p. 20. 



Lower Silurian Cerrig-y-Druidion. 



5. „ Ji^orSesi, Hall, Upper Silurian, Herkimer Co., New York. 



XI. Ptilonaster princeps, Hall, 1866, 20th Eeport, p. 12, pi. ix. fig 9. Chemung 

 Group, Cortlandville. 

 XII. 1. Stenaster Salteri, Billings, Geol. Surv., Canada, Org. Eems. Decade iii. 

 p. 78, pi. X. fig. 1. Trenton Limestone. Belville, Canada West. 

 2. „ Huxleyi, Billings, Quebec Group, Lower Silurian, Newfoundland. 



XIII. 1. Twniaster spinosus, Billings, Canadian Organic Eemains, Decade iii., pL 



X., fig. 3, p. 81. Trenton Limestone ; Falls of Montmorency. 

 2. „ cylindricus, Billings, op. cit. pi. x., fig. 4, p. 81. Trenton Lime- 



stone, Ottawa City. 



XIV. Urasterella (Stenaster) pulckella, Billings, sp. Geol. Surv. Canada, Report 



1856, p. 292. Hall, 20th Eeport on State Cabinet, 1866, p. 9. 

 Trenton Limestone, Ottawa City, Canada West. 



Giving a total of 14 genera and 49 species of Silurian Star-fistes, 



18 of whicK are British and the others (with three exceptions only) 



are North American. 



EXPLANATION OF PLATE VIII. 



Fig. 1 a. Eucladia Johnsoni, sp. nov. Upper Silurian, Sedgley, near Dudley. 



Drawn from the original specimen in the Cabinet of Henry Johnson, 



Esq., Dudley (natiiral size). 

 „ 1 5. The Madreporiform plate enlarged three times natural size. 

 „ 1 c. Three of the body-plates ,, „ „ „ „ 



„ 1 d. Part of one of the arms „ „ „ „ „ 



II. — On the Probable Seat op Volcanic Action. 

 By T. Sterrt Hunt, LL.D., F.R.S. 



THE igneous theory of the earth's crust, which supposes it to have 

 been at one time a fused mass, and to still retain in its interior 

 a great degree of heat, is now generally admitted. In order to ex- 

 plain the origin of eruptive rocks, the phenomena of volcanos, and the 

 movements of the earth's crust, all of which are conceived by geolo- 

 gists to depend upon the internal heat of the earth, three principal 

 hypotheses have been put forward. Of these the first supposes that 

 in the cooling of the globe a solid crust of no great thickness was 

 formed, which rests upon the still uncongealed nucleus. The second 

 hypothesis, maintained by Hopkins and by Poulett Scrope, supposes 

 solidification to have commenced at the centre of the liquid globe, 

 and to have advanced towards the circumference. Before the last 

 portions became solidified, there was produced, it is conceived, a 

 condition of imperfect liquidity, preventing the sinking of the cooled 

 and heavier particles, and giving rise to a superficial crust, from 



