74 BRITISH FOSSIL CORALS. 



Since the publication of our Monograph of the Family of Astreid^ (Annales des 

 Sciences Naturelles, s. 3, vols, xi 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. xii), and in the Conspectus forming the Introduction to our Monograph of the Palaeozoic 

 Corals, we have designated this new division by the name of Isastrea. In the genus 

 Prionastrea, as now circumscribed, the walls are double in the lower part of the corallum ; 

 in Isastrea they are always simple. In the latter the columella is rudimentary or does not 

 exist, and the septa 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 Isastrea have as yet been met with only in 

 secondary deposits. 



The fossil here designated by the name of Isastrea oljlonga 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. Ic) 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, in consequence of the primary septa 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 cyclnm 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 septa arc 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 apai-t; some remain simple, but most of them become bifurcate inwards. 



