74. BRITISH FOSSIL CORALS. 
Since the publication of our Monograph of the Family of Asrrem#® (Annales des 
Sciences Naturelles, s. 3, vols. x1 and xii, 1849), and that of the First Part of this Work, 
we have been led to consider, as being of generical value, a group composed of most of the 
species which we formerly placed in the Second Section of our genus Prionastrea (Introd., 
p. xi), and in the Conspectus forming the Introduction to our Monograph of the Palaeozoic 
Corals, we have designated this new division by the name of Isasvrua. In the genus 
Prionastrea, as now circumscribed, the walls are double in the lower part of the corallum ; 
mn /sastrea they are always simple. In the latter the columella is rudimentary or does not 
exist, and the sep/a are terminated by a crenulated edge, the denticulations of which are of 
nearly equal size ; in Prionastrea the columella exists, and the marginal denticulations of 
the septa increase in size from the circumference of the corallum towards its centre. These 
differences are shown in some well-preserved specimens which we have but lately had the 
opportunity of examining, and it may be worth noticing, that the two generical groups thus 
separated appear to have each a distinct geological range; all the true Prionastrea being 
either recent or tertiary species, whereas Jsastrea have as yet been met with only in 
secondary deposits. 
The fossil here designated by the name of Jsastrea oblonga is very common in the 
Portland beds of Tisbury, Wiltshire; it is a massive corallum, completely silicified, and 
when polished shows its characters in a very distinct manner. By means of a horizontal 
section (fig. lc) it is easy to see that the corallites are circumscribed by simple, thick 
walls of a pentagonal or hexagonal form; that the columella is quite rudimentary, if not 
completely deficient ; and that the principal septa reach quite to the centre of the visceral 
cavity, but do not join together by their inner edge, and are united only by means of small 
trabicula, which occupy the place usually filled by the columella. The six septal systems 
are in general very distinct, im consequence of the primary sepéa being much more deve- 
loped than those of the following cycla, and two of these systems are much larger than 
the four others. The septa form four complete cycla, but those of the last cyclum are 
rudimentary in the four small systems above mentioned; they are all nearly straight, 
somewhat thick, and strongly granulated laterally; they are very unequal in size in the 
different cycla, and it often happens that those of the fourth cyclum are more developed 
in one half of each system than in the other, and in that case the tertiary septa situated 
between the former, are also somewhat more developed than those of the other half 
systems ; but the secondary sepéa are all nearly of equal size, and even those of the two 
large systems are never as much developed as the primary ones, which alone reach to the 
centre of the visceral chamber, and become rather thicker internally. 
A vertical section (fig. 1/) shows that the inner edge of the septa is delicately and 
almost regularly denticulated. The dissepiments, which in many specimens have disappeared 
completely, or have been more or less modified in form by the process of fossilisation, are 
well developed, arched, somewhat decline inwards, and situated at one third or one fourth 
of aline apart ; some remain simple, but most of them become bifurcate inwards. 
