DISCINID &. 71 
the accompanying illustrations, to give the external characters by means of considerably 
enlarged figures. 
Position and Locality. Sowerby’s original specimen, in the Museum of the Geo- 
logical Society, is stated to have been obtained from the Caradoc beds of Chatwall, on 
the east flank of Caradoc. It occurs in the same rock at Cheney Longville Lane, Shrop- 
shire, and at Bowdler, near Church Stretton, in the same county. Specimens showing 
portions of the sculptured shell may be seen in the Museum of the Geological Survey. It 
is most commonly found in the condition of casts. Mr. Lightbody has obtained the shell 
in the Caradoc sandstone of Onny, at Marshbrook, and in several other localities. No 
perfect example has, however, been hitherto discovered. 
DIscrna (Trematis) Srnurtana, Dav. Pl. VI, fig. 8. 
Spec. Char. Orbicular, posteriorly slightly acuminated, wider than long; upper or 
free valve depressed or very moderately convex, most so in the proximity of its umbonal 
extremity, the apex being submarginal or close upon the posterior: margin. External 
surface covered with close-set, raised, flattened, thread-like ridges, radiating from the 
apex and increasing in number near the margin by an additional small interpolated 
ridge along the centre and between each two of the original ribs. The concave 
interspaces left between the radiating ridges are occupied by a succession of small trans- 
versely oval cells, which are encircled, or separated from each other, by smaller transverse 
ridges or bars. The cells are slightly concave, but do not penetrate further than the 
surface of the shell. Interior unknown. 
Length 7, width 8, depth 2 lines. 
Obs. Of this beautifully sculptured shell I am acquainted with several upper valves ; 
the lower or perforated valve has not been hitherto discovered. In one example, around 
the middle of the shell I have counted as many as one hundred principal radiating 
ridges, while near the margin the number had increased to 180; the interspaces 
between each of the principal pair of ridges was also divided into sixty or seventy 
consecutive oval cells, separated from each other by smaller ridges. At and close 
to the vertex the shell is smooth, the cells commencing by a succession of small 
dots, which become gradually larger and more defined as they near the middle of the 
valve; from that point they again become smaller and smaller as they approach the 
margin. One cannot, therefore, say that the radiating ridges, or rather the interspaces 
between them, are crossed by concentric elevated lines, because the smaller ridges 
separating the cells are independent, and not on the same line or level with those in the 
adjoiing rows (fig. 8 4). 
In the Museum of the Geological Survey, London, a very fine example of the shell 
under description has been labelled Discina (Trematis) cancellata ; but after an attentive 
