ASTREA. | 263 
The corals of this genus are very numerous in the seas of 
_ the Tropics, and there are many species in the Oolite, and 
older secondary’ formations. The Astrez, Caryophylliz, 
_ Cyathophylla, &c., form the principal mass of the coralline 
_ limestones of the Oolite, termed the Coral-rag, from the 
abundance of these relics: being literally composed of an 
ageregation of large corals, the interstices of which are filled. 
with shells, radiaria, &c., either whole, or in a comminuted 
state. The heaps of this limestone placed by the road-side, 
in the N. W. of Berkshire, appear like fragments of an old 
- coral-reef, and attract the notice even of the most incurious 
observer. I have figured a specimen of Astrea, Lign. 88, 
jig. 1, and a polished section, jig. la, from Clifton, a locality 
well known for the stupendous mural precipices of mountain 
limestone rocks, which yield beautiful examples of coralline 
marble.* The mode of increase of the Astrea is very curious ; 
a subdivision takes place in the old cells, after the manner 
of the Infusoria ; and among the fossils, a star or cell may 
often be seen in progress of division into two, three, or four 
stars (Si. Syst. pl. xvi. fig. 6). A living polype of this genus 
is figured, Wond. pl. vi. fig. 13. 
A species of Astrea (A. Tisburiensis. Wond. p. 641, fig. 9), 
is found in large hemispherical masses, completely silicified, 
at’ Tisbury, in Wiltshire. The transverse surface displays, 
in some specimens, beautiful white radiated stars, on a dark 
blue ground ; and in others, the colours of the stars and 
ground are reversed. ‘This silicified coral is obtained from 
a bed of chert, a foot in thickness, which is interstratified 
with the Portland limestoue, this division of the Oolite being 
quarried around Tisbury. 
* Astrea arachnoides, Pict. Atlas, pl. xxxviii. fig. 4. 
undulata, ibid. pl. xxxvili. fig. 10. 
Tisburiensis ibid. pl. xxxviii. figs. 12, 13. 
+ See Catalogue of the Organic Remains of Wiltshire, p. iv. by 
Miss Etheldred Benett. 4to. 1831. 
