CORALS FROM THE SILURIAN FORMATION. 251 



them being, in general, equal to twice, and sometimes to thrice their diameter. The 

 polygonal divisions of the ccenenchyma are very regular in form and in size ; their breadth 

 is about half of a line. 



A vertical section of this fossil shows that the walls are very distinct, and the tabulae 

 closely set, and in general placed horizontally, but sometimes slightly oblique. The 

 appearance of the ccenenchyma differs in the upper and lower parts of the corallum ; near 

 the upper surface, to the depth of about 1 line, the vertical lines corresponding to the 

 section of the prism atical lamina of the tubes, are much more strongly marked than the 

 horizontal lines that correspond to the sections of the trabiculae or small intratrabicular 

 diaphragms ; but in the lower parts of the corallum a contrary disposition exists, and the 

 transversal lines running nearly parallel to the surface of the compound mass, are the only 

 ones which are distinct to the naked eye. 



H. Murchisoni is found in the upper Silurian deposits of Wenlock Edge. Professor 

 M'Coy has met with it at Aymestry, in Herefordshire ; at Coniston Waterhead, in Lan- 

 cashire ; and at Llansantfraid, in Denbighshire. It is found also in Gothland. 



Specimens are in the Collections of the Parisian Museum, of the Museum of Practical 

 Geology of London, of M. de Verneuil, and M. Michelin. 



This fossil coral differs from H. interstincta 1 by the greater distance between the calices, 

 and the abundance of the ccenenchyma. It much resembles H. porosa" by its external 

 characters, but differs from it by the structure of its ccenenchyma ; the horizontal laminae 

 of that tissue being much more developed than the vertical ones, whereas the contrary 

 takes place in II. porosa. 



3. Heliolites megastoma. Tab. LVIII, figs. 2, 2a, 23, 2c, 2d. 



Porites pyriformis (pars), Lonsdale, in Murchison, Silur. Syst., p. 686, pi. xvi, figs. 2d, 2e 



(cast, excl.), 1839. 

 Porites megastoma, M'Coy, Silur. Foss. of lrel., p. 62, pi. iv, fig. 19, 1846. 

 Geoporites intermedia, UOrbigny, Prodr. de Pal., vol. i, p. 49, 1850. 

 Pal^eopora megastoma, M'Coy, Brit. Palseoz. Foss., p. 16, pi. i c, fig. 4, 1851. 

 Heliolites megastoma, Milne Edwards and Jules Haime, Pol. Foss. des Terr. Palseoz. 



(Arch, du Mus., vol. v), p. 216, 1851. 

 Heliolites macrostylus, J. Hall, Paleont. of New York, vol. ii, p. 135, pi. xxxvi a, 



fig. 2, 1852. 



Corallum composite, massive, irregularly hemispherical; sometimes adherent, sometimes 

 free. Epitheca of the under surface not much developed. Calices having all nearly the same 

 dimensions in the same specimen, but varying from two thirds of a hue to \\ line in different 

 specimens ; very closely set, quite circular, and with a margin not prominent, and scarcely 



1 See tab. lvii, fig. 5. 2 See tab. xlvii, fig. 1. 



