94 . BRITISH FOSSIL CORALS. 



Genus Isastrea. 



1. Isastrea explanata. Tab. XVIII, figs. I, \a, \b, \c, \d. 



Madrepora, W. Smith, Strata identified by organic fossils, p. 20, Coral Rag, fig. 1, 1816. 

 Compound madrepora, G. Young, Geol. Survey of York, tab. iv, fig. 2, 1828. (Very rough 



figure.) 

 Astrea, approaching to A. Favosa, IF. D. Conybeare and W. Phillips, Geol. of England, 



p. 188, 1822. 

 Astrea explanata, Goldfuss, Petref. Germ, vol. i, p. 112, tab. xxxviii, fig. 14, 1829. 



— favostoiues, Phillips, Illustr. of the Geol. of Yorkshire, vol. i, p. 126, tab. iii, 



fig. 7, 1829. 

 SxDERASTREA EXPLANATA, Blainville, Dict. des Sc. Nat., v. Ix, p. 337, 1830: and Manuel 



d'Actinologie, p. 371. 

 Astrea explanata, Milne Edwards, Annot. to Lamarck, vol. ii, p. 420, 1836. 



— — ^TOwn, Lethea Geognostica, vol. i, p. 299, 1837. 



— helianthoides, M'Coy, Ann. of Nat. Hist., s. ii, vol. 2, p. 408, 1848. 

 Pkionastrea explanata, Milne Edwards and J. Haime, Monogr. des Astreides, Ann. des 



Sc. Nat., s. iii, vol. xii, p. 136, 1849. 



Coralhm composite, massive, and convex. The common basal plate covered with a 

 complete epitheca, which is often partially worn away, and then leaves exposed to view 

 the costal striae. They are narrow, somewhat unequal in size, and arranged in fasciculi that 

 radiate from the basis to the circumference of the basal plate, so that the outer striae of 

 each group meet those of the neighbouring fasciculi under a very acute angle (fig. I), The 

 calices are in general polygonal and very unequal in size, especially in large specimens. 

 They are shallow, and present in their centre a small round fossula, at the bottom of Avhich 

 is a rudimentary columella. Sometimes these small fossulae become filled up with extra- 

 neous stony matter, that assumes the appearance of a prominent columella (fig. \d). The 

 edges of the calices are convex, and intimately united together. Sometimes the septa of 

 one calice appears even to extend without interruption into the adjoining calice ; but in 

 general the corallites are circumscribed by a very delicate mural line or a narrow furrow. 

 The septal systems are rather irregular ; the first three cycla are complete ; the fourth 

 cyclum more or less incomplete, and the total number of septa thus varies from twenty- 

 eight to forty-four. The septa are broad, thin towards their outer edge as well as inwards, 

 closely set, often flexuous, and but slightly exsert ; their upper edge is almost straight, 

 descends obliquely towards the fossula, and is divided into a series of small, closely set, 

 and nearly equal denticulations, each of which corresponds to a series of granulations 

 situated on the lateral surfaces of the septum. The secondary septa are almost as large 

 as the primary ones, but their inner edge does not ascend so high ; the tertiary ones are 

 much smaller, and those of the fourth cyclum still less. The greatest diagonal of the 

 adult individuals is in general about four lines, but varies much in the different parts of 

 the same specimen. 



1 See page 74. 



