A SUMMER RAMBLE AMONG THE HEBRIDES. ] 45 



nothing can be more irregular than the outline of each mass, 

 and vet scarce anything more regular than the sculpturings 

 on every part of it. We find them fretted over with poly- 

 gons, like those of a honeycomb, only somewhat less mathe- 

 matically exact, and the centre of every polygon contains its 

 many-rayed star. It is difficult to distinguish between species 

 in some of the divisions of corals : one Astrea, recent or ex- 

 tinct, is sometimes found so exceedingly like another of some 

 very different formation or period, that the more modern might 

 almost be deemed a lineal descendant of the more ancient 

 species. "With an eye to the fact, I brought with me some 

 characteristic specimens of this Astrea* of the Lower Lias, 

 which I have ranged side by. side with the Astreoe of the 

 Oolite I had found so abundant a twelvemonth before in the 

 neighbourhood of Helmsdale. In some of the hand speci- 

 mens, that present merely a piece of polygonal surface, bound- 

 ed by fractured sides, the difference is not easily distinguish- 

 able : the polygonal depressions are generally smaller in the 

 Oolitic species, and shallower in the Liasic one ; but not un- 

 frequently these differences disappear, and it is only when com- 

 pared in the entire unbroken coral that their specific pecu- 

 liarities acquire the necessary prominence. The Oolitic As- 

 trea is of much greater size than the Liasic one : it occurs 

 not unfrequently in masses of from two to three feet in dia- 

 meter; and as its polygons are tubes that converge to the 

 footstalk on which it originally formed, it presents in the 

 average outline a fungous-like appearance ; whereas in the 

 smaller Liasic coral, which rarely exceeds a foot in diameter, 

 there is no such general convergency of the tubes ; and the 

 form in one piece, save that there is a certain degree of flat- 

 ness common to all, bears no resemblance to the form in ano- 

 ther. Some of the recent Astreae are of great beauty when 

 inhabited by the living zoophites whose skeleton framework 



* Probably one of the Tsastrea of Edwards. 

 K 



