94. BRITISH FOSSIL CORALS. 
Genus IsastREA.! 
1. Isasrrea ExpLanaTa. ‘Tab. XVIII, figs. 1, la, 14, le, 1d. 
Maprepora, WW”. 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, 1528. (Very rough 
figure.) 
ASTREA, approaching to A. Favosa, W.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. 
—  ravosrorpes, Phillips, Mlustr. of the Geol. of Yorkshire, vol. i, p. 126, tab. iii, 
fig. 7, 1829. 
SIDERASTREA EXPLANATA, Blainville, Dict. des Sc. Nat., v. lx, p. 337, 1830: and Manuel 
d’Actinologie, p. 371. 
ASTREA EXPLANATA, Milne Edwards, Annot. to Lamarck, vol. ii, p. 420, 1836. 
— — Bronn, Lethea Geognostica, vol. i, p. 299, 1837. 
— HELIANTHOIDES, M‘Coy, Ann. of Nat. Hist., s. ii, vol. 2, p. 408, 1848. 
PRIONASTREA EXPLANATA, Milne Edwards and J. Haime, Monogr. des Astreides, Ann. des 
Sc. Nat., s. ili, vol. xil, p. 136, 1849. 
Corallum 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 strize of 
each group meet those of the neighbouring fasciculi under a very acute angle (fig. 1), The 
calices are m general polygonal and very unequal in size, especially m large specimens. 
They are shallow, and present in their centre a small round fossula, at the bottom of which 
is a rudimentary columella. Sometimes these small fossula become filled up with extra- 
neous stony matter, that assumes the appearance of a prominent columella (fig. 1d). 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 adjoming calice; but m 
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 immer 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. 
