170 PROF. P. M. DUNCAN ON STREPTELASMA RG@MEBPI. 
The height of the ye is from 3 to 2 inch, and the breadth 
of the calice is from 58, to 3 inch. 
Locality. Wenlock Shale ; common. 
Remarks, The variability of the septal arrangement in different 
specimens in which that of the cost is constant is very striking ; 
moreover, the vertical pair of cost, and those at the lateral quad- 
rants of the calicular marginal circle, vary in a most irregular 
manner in their relation to the fossula, to large and small septa, 
and to the septal pinnation. 
In some individuals the vertical pair of coste at the calicular 
margin are at the fossula, and then the long septum is opposite, 
and the pinnate half of the calices commences at the quadrants (a). 
This is the normal condition; but it is by no means constant. 
For a long septum may replace the fossula, or the whole septal 
arrangement may be out of its normal condition, and there is no 
relation between the vertical pair of coste and special septa (fig. 14). 
Again, the thickness of the wall varies: in some it resembles an 
epitheca, and in others it is a stout union of septal and costal bases 
with intermediate tissue. 
The relation of the intercostal space to the median line of the 
septum, seen in several instances, is very interesting (fig. 8). It is 
rare in corals usually, but it is seen very constantly in Australian 
Cainozoic Aporosa. The development of the septum occurs from 
the sides of a median plane of connective tissue in these instances, 
and this tissue must have been continuous with the epitheca out- 
side the corallum. 
Ordinarily the relation of costa and septum is that the latter is 
continuous with the former, and then there is no connexion between 
the mesenteries and the extramural soft parts (figs. 2, 7). 
The costulation and the septal arrangement of the species connect 
it with the genus Streptelasma of Hall; but it is slightly aberrant 
on account of the defective obtortness of the septal ends and the 
presence usually of a well-marked fossula in the calice. But the 
method of septal junction, the presence of the rod-like tissue of the 
false columella, and the rarity of dissepimental structures, all seen 
in the species, are very characteristic of this genus. 
Compared with Streptelasma europeum of Rémer*, the new form 
is noticed to differ on account of its septal fossula and extremely 
defective tabule and dissepiments. The number of the septa and 
their arrangement, and the presence of the very distinct double 
pinnation of the coste and of the vertical pair are very distinctive. 
The genus was = and hardly sufficiently diagnosed by Hall 
in his magnificent ¢ Paleontology of New York’ (1847, vol. i. p. 17), 
and he relied too much upon the value of the obtort septal ends in — 
classification. MM. Milne-Edwards and Jules Haime introduced 
the genus into their ‘ Histoire Naturelle des Coralliaires’ (vol. ii. 
p. 392). They did not consider the twisted septa of generic im- 
portance, as it is a well-known character of some species of Cyatho- 
* Silurische Fauna von Sadewitz, p. 16, pl. iv. fig. 1. a, 6 (1861). 
