CARBONIFEROUS BRACHIOPODA. 
297 
" The two series of perforations are well illustrated in a small variety of the shell from 
Bowertrappiiig Limestone in Ayrshire. When the ^hoWoi Prod._(ji(/anteus 'i's, decorticated 
or stripped of its outer layer, these perforations, large and small, are seen to project from 
the surface of the decorticated layer as conical or funnel-shaped tubercules. The larger 
of these tubercles are well illustrated in the figure you give of the large Prod, (jigantem 
in Pi. XXXVIII of your Carboniferous Monograph, and, like that specimen, all the 
examples I have seen show these tubercules larger and more conspicuous on the 
auricular expansions of the shell. There is a curious feature that I have noticed in 
Prod, (jiganteiis, as well as in several other species of the genus, and which I do not 
think has yet been remarked upon : it is this — the shell has always a great tendency 
when extracted from hard limestone to part or break away near the point where the 
perforated inner layer comes in contact with the non-perforated outer layer, and this 
peculiar character is also well illustrated in your PI. XXXVIII, where the inner and 
outer layers of the shell still remain on the same specimen. Having observed this 
character, as before stated, in numerous specimens extracted from hard limestone, it 
seems to show that the layer of shell between the perforated inner portion, and that of 
the non-perforated outer portion is its weakest line, and causes the outer portion to 
remain in the hard matrix, while the inner portion remains attached to that filling 
the shell. 
" 2. Proditdus latissimus. — An example of this shell from the Robroyston Limestone at 
Solesgirth, near Kirkintulloch, shows on the decorticated outer portion of shell numerous 
small tubercules which are the termination of perforations passing outward from the 
inner surface of the ventral valve, but which, as clearly seen, do not reach the outer 
layer of the shell. This is the only example of the species I have found showing the 
perforations, which, I may mention, incline to arrange themselves into irregular lines 
corresponding somewhat to the ribbing of the shell. 
" 3. Producttis semireticulatus (Sup., PI. XXXVI, fig. 17). — Examples of this species 
and its varieties show that the inner layer of the shell in both valves is penetrated by 
numerous small perforations that are quite visible to the eye with an ordinary pocket lens. 
Partly decorticated specimens from limestone, and specimens from shale that have had the 
outer shell removed by acid, clearly show that the perforations do not reach the outer layer 
of the shell. The perforations are very numerous all over the inner surface of the valves, 
with the exception of the muscular and reniform impressions ; and this seems also to be 
the case in other species wherever the interiors are well preserved. On decorticated 
specimens, the perforations exist on the surface of the shell as small raised tubercules, 
which gives the shell a minutely roughened appearance when looked at under the glass, 
and they are also seen to be somewhat irregular in their arrangement over the surface. 
In some specimens of this species, the frontal portions of both the ventral and dorsal 
valves seem to have been adorned with two or three irregular rows of small external 
tubular spines that passed through the entire thickness of the shell and are continued on 
