25 



The top-like madrepore (madrepora turbinata) Plate IV. Fig. 1, 2, 

 3, &c. smooth, with a concave hemispheric star, is a zoophyte, onlj 

 known in a mineralized state. 



This species of madrepore has in general a conical figure, the lower 

 part of the cone being often very much contracted, as if to form a 

 pedicle. From this point, the body of the fossil gradually enlarges, 

 until it terminates in a truncated apex ; or is previously prolonged 

 into a cylindrical oblong body. In either case, the surface is divided 

 by numerous, small, and hardly perceptible longitudinal striae, and 

 girt with transverse, obtuse, and unequal furrows. 



The star which fills the apex is concave, and formed by sulcated, 

 or lamellated, and frequently even, dentated rays, proceeding from 

 the central depression to the edge of the cavity, which is generally 

 acute. 



These fossils are commonly formed of a lime-stone, which, in general, 

 is internally, of a light, or yellowish brown colour ; but they are mostly 

 externally of a bluish grey, deriving this colour from the matrix in 

 which they have been imbedded. 



These madreporites vary much in size; some hardly exceeding that 

 of a horse-bean ; whilst others are three or four inches in length, .and 

 others of even nearly that diameter. 



No investigation respecting these fossil substances has been con- 

 ducted with so much success as that which has been instituted by the 

 learned author of the Dissertation on the Baltic -Corals. Nor can the 

 figures with which he illustrates the several varieties of this species of 

 madreporite be exceeded, in exactness of representation. To atone, 

 therefore, to Ariose who have it not in their power to refer to those 

 figures, for their repetition here, I have, as I have in every -other 

 parallel case, endeavoured to select such specimens for the engraver, 

 as may serve also to point out some important circumstance to which 

 the figures in former works do not refer. 



VOL, II. 



