159 
G. W. Lee — Trepostomaia. 
equal to about one-third of the diameter of the zooecial cavity. Tabulae are very scarce ; a 
few are developed in the outer portion of the peripheral region, very rarely elsewhere. 
The ratio of axial region to diameter is equal to 0*5 : 1. 
Tangential sections show that a cell smaller than the average occasionally occurs in 
the junction-angles. The acanthopores are small. In shallow section about eight are 
seen round each zooecium ; in deep section they are fewer. They evince no tendency to a 
regular disposition according to size, but rather to an arrangement in two sets, the one 
deeper than the other, though of the same size. 
Distribution.— This species is so far known from one locality only, namely Treak 
Cliff, Castleton, Derbyshire, in beds of D 2 age (top of the Yisean), approximately. 
Observations. —The most characteristic feature of Stenopora castletonensis is the 
tenuity of the zooecia relatively to the size of the zoarium. This character distinguishes 
it from most of the other Stenoporae that have come under observation. The only ones 
somewhat comparable in this respect are Stenopora obliqua sp. nov. and Stenopora 
tenuipora sp. nov., which are distinct in having still smaller zooecia and different wall- 
structure and tabulation. 
The specimens on which the present species is founded were collected by Dr. T. F. 
Sibly, who kindly presented them to the Geological Survey., . 
Stenopora obliqua sp. nov. 
Plates XIV, fig. 12 ; XY, fig. 5. 
Description. —(a.) External characters. The zoarium is composed of cylindrical 
branches 2 to 5 mm. thick, which may exceed 12 cm. in length. The branches are either 
hollow or solid, the two modes of growth co-existing in the same bed. The surface is 
not well shown in the specimens at hand, but it appears to be devoid of maculae or 
monticules. There are some sixty apertures along a distance of 1 cm. Fractured 
specimens show that the zooecia curve gradually from the axial region to the surface, on 
which they fall at an acute angle. 
(b.) External structure. As seen in longitudinal sections, the zooecia-walls are very 
thin in the axial region. In the peripheral region they reach a thickness equal to about 
two-thirds of the diameter of the zooecial cavity. They show very little tendency to 
periodical cessations of thickening, but are often appreciably flexuous. Tabulae are scarce 
in the axial portion of the tubes, but numerous in the peripheral region and in the 
222S9 D 
