EXPLANATION OF THE PLATES AND WOOD-CUTS. 



stances by vertical filaments. The horizontal section (Fig. 2.) presents more or 

 less perfectly, perforated centres, from which irregular tubes diverge, also the 

 terminations of the filaments, and portions of the concentric layers. 



Loc. Appleway and near Teignmouth. 

 Fig. 3, 3 b. Astrea Hennahii (Sp. n.). 



Stars not defined either on a weatliered surface (Fig. 3.) or in a polished section 

 (Fig. 3 «.). On the former, the centre of the star presents a radiated boss, in the 

 latter, fine converging lines. Rays about thirty-six, unequal in length and breadth, 

 and of a crenulated structure. The rays of one star unite with those of the ad- 

 joining stars, the junctions being more or less regular according to their position 

 with respect to the angles of the stars. The rays are alternately long and short, 

 the latter terminating around the central portion of the star, and the former which 

 swell out at the same boundary suddenly become attenuated beyond it, and are 

 prolonged as fine laminae to a reticulated centie. The vertical section (Fig. 3.) is 

 composed of perpendicular close-set parallel lamellae united by innumerable trans- 

 verse plates (Fig. 3 b.). 



Loc. Barton Quarry, Newton, Plymouth. 

 Fig. 4, 4y. Porites pyriformis, Ehrenberg. 



Astreaporosa (Goldfuss, 21, f. 7.) Porites pyriformis (Silur. Syst. PI. XVI. f. 2.). 



This fossil abounds in the Devonian limestones as well as in the Silurian system ; 

 but specimens of the perfect coral, or in that state in which it is presumed the 

 polype ceased to add to its strong fabric, appear to be very rare. I have seen 

 only a few belonging to Mr. Austen's collection. In the young state, and during 

 the period when additions were made to its vertical dimensions, the coral con- 

 sisted of circular tubes, with a projecting margin, from which twelve rays pro- 

 ceeded and united in the centre ; and the interstices between the circular tubes 

 were occupied by smaller polygonal ones, open at the upper end. In the state in 

 which it is supposed the polype ceased to add to the structure of the coral, the 

 fossil (Fig. 4, 4 a. 4 c. 4 d. 4 e.) presents in the place of the circular depressed 

 tubes, a raised boss, the margin of which is a sunken and not a projecting line ; 

 and the intervals between the twelve rays of the immature coral are occupied by 

 convex surfaces, separated by fine furrows. These bosses are surrounded at irre- 

 gular distances by polygonal ridges, giving the coral the aspect of a series of stars 

 in close contact (Fig. 4.). The ridges and the whole of the surface, which in the 

 young specimens consist of open tubes, are formed of convex papillae preserving in 

 greater or less perfection the angular form of the tube. 



The right half of Fig. 4 c. exhibits the usual character of the coral, except that 

 the projecting margin of the circular tubes is wanting ; the left half of the same spe- 

 cimen presents imperfectly the character of the mature coral. Fig. 4 d. has been 

 drawn from a partly polished specimen, the remainder giving examples of the radi- 

 ated boss, but without the bounding polygonal ridges. Fig. 4/. is a magnified 

 representation of the polished surface, and Fig. 4 e. of the bosses. Fig. 4 exhibits 

 the character of the perfect coral, and Fig. 4 a. is a magnified portion of the same 

 specimen. Fig. 4 b. displays the internal structure of the fossil. 



