120 Prof. H. A. Nicholson’s Contributions 
view in describing his Heteropora crassa. As regards its 
general characters, it is distinguished from F’. crassa by its 
habit of growth, and also by the much greater development of 
the interstitial tubuli, which give to the surface of well-pre- 
served specimens a minutely porous appearance. Moreover, 
the mouths of the corallites are not surrounded by prominent 
rims. As regards internal structure, the chief features which 
distinguish F. ludensis from F. crassa are the greater number 
of the interstitial tubes and their more complete walls, the 
smaller size of the ordinary corallites and their more com- 
plete isolation, and the presence of well-marked “ spiniform 
tubuli.”” 
Formation and Locality. Wenlock Limestone, Dudley. 
The best preserved specimen I have seen forms a thin crust 
growing upon a specimen of Monticulipora pulchella, K. & H., 
which it entirely envelops. 
3. Callopora nana, Nich. (Pl. VII. figs. 4-4 d.) 
The corallum in this species is in the form of minute, 
cylindrical, or bulbous masses, generally two or three lines 
in length, and about a line or a line and a half in dia- 
meter. The surface is free from monticules or macule, but 
exhibits the openings of the large circular or oval coral- 
lites, largely or wholly separated by regular, often oblong 
interstitial pores. In tangential sections (Pl. VII. fig. 4 a) 
the corallites are seen to be oval or subcircular, averaging 
about = inch in their long diameter, which corresponds 
in direction with the long axis of the corallum. They are 
separated by intervals occupied by the interstitial tubes, which 
have quite complete walls, and are mostly long-oval or irre- 
gularly oblong in shape. The long diameters of the intersti- 
tial tubes correspond with the long axis of the corallum, and 
vary from 74, inch to #5 inch, their shorter diameters being 
from +35 to 74, inch. Hence the intervals separating con- 
tiguous corallites are much greater in the direction of the long 
axis of the coral than when measured transversely to the 
corallum. 
In long sections (Pl. VII. fig. 4 6) the corallum is seen to 
be composed of tubes which are vertical in the axis of the 
colony, and then gradually bend outwards to open on the 
surface. They are similar in internal structure through- 
out their entire extent, complete horizontal tabule bemg 
largely developed both in the axial region .and the peripheral 
region, while their walls have a nearly uniform thickness 
throughout. As they bend outwards, however, towards the 
