R. F. Tomes—On Heterastreea, Lower Lias. PAI 
united by their walls, but the union is often incomplete, and the line 
of contact frequently visible. 
There is an occasional trace of a common or basal wall, with a 
rudimentary pellicular epitheca, both of which, however, are very 
often wanting. 
The endotheca consists of dissepiments which are not vesicular, 
but flat and more or less tabular, and there are occasionally distinct 
tabule, many of them passing quite across from wall to wall of the 
corallite, forming a level floor to the calice. 
Increase takes place by gemmation, which is strictly marginal, and 
never calicinal, and by fissiparity, which is first indicated by the 
elongation of the dividing calices, followed by the appearance of 
two or more fossule, which are finally divided from each other by 
the union of two elongated septa.’ 
The prevalence of gemmation over fissiparity, or the reverse, 
exercises a decided influence, not only over the shape of the calices, 
but also to some extent over the general contour of the corallum. 
The elevation of budding calices above the level of the surrounding 
ones tends to produce a more gibbous surface than does the division 
of the calices by fission, and the conformation of the corallum is 
modified accordingly. This will be further noticed when I speak of 
the species, which I shall now proceed to do, first enumerating 
those which I have not had the opportunity of examining. They 
are :—Isastrea intermedia, De Ferry; I. excavata, De Ferry; I. 
basaltiformis, Fromentel ; I. Orbignyt, Chap. et Dewal.; I. Condeana, 
Chap. et Dewal.; I. Moreneyana, Terq. et Piette; I. ? Henacquei, Kd. 
et Haime; and Latimgandra denticulata, Duncan, all of which have 
been stated to occur in the Lower Lias. 
HererastR#A Mourcuisoni, Wright sp. 
Isastrea Murchisoni, Dunc., Supp. Brit. Foss. Cor. pt. iv. p. 41, pl. xi. figs. 1-4. 
The only example of this remarkable species, from the Isle of 
Skye, to which I have access, is small in size, and has but little 
calicular surface. Yet, small as it is, it has marginal gemmation, 
and elongated calices which, having more than one fossula, with an 
indication of dividing septa between them, affords indubitable 
evidence of fissiparity. Prof. Duncan says, ‘There is often a ridge 
between the margin of the calice and the centre, indicating calicinal 
gemmation, but the gemmation usually takes place at the margin, 
and there is no fissiparity.” The ridge here mentioned has nothing 
to do with gemmation, but is merely the commencement of rejuve- 
nescence, and in one calice of my specimen it has proceeded so tar as 
' Tt is necessary to notice here a variation in the process of splitting that takes 
place in different calices, or even in the same calice. The two opposite and approach- 
ing septa do not always meet and make the division until a new calice has been 
actually formed, and it is common for two or three fossulee to make their appearance 
in a long calice with only some, or even without any, dividing septa. When this is 
the case, there is some resemblance to Latimeandra. However, the septa always 
become elongated sooner or later, and fissiparity is then complete. M. de Fromentel 
gives a good account of the process in his description ot the so-called Septastrea 
excavata. 
