296 
SUPPLEMENT TO THE ERmSII 
tion, the correctness of which I can fully substantiate, having myself carefully studied 
the material upon which his statements are founded. 
On March the 9th Mr. Young writes me, " I have now found perforations in twelve 
or thirteen species of my Scottish Carboniferous Produdi. The first point I have 
satisfied myself about is that the perforations in the several species are only to be seen 
on the inner layers of the shell, and that in no instance, except doubtfully in Prod, 
mesolobus, do they show themselves on the outer surface of the shell. The second point 
is that the perforations enter from the interior of the valves aud gradually get smaller 
and smaller as they pass outwards through the substance of the shell, and are lost before 
they reach the outer surface, so that it is only in the interior surface of the ventral and 
dorsal valves that these perforations are seen, or where the outer layers of the shell are 
decorticated or stripped off, as is often the case where the specimens are extracted from 
hard limestone. When the perforations are seen in this latter condition the substance of 
the shell rises around the perforations, and they then appear on the decorticated surface 
of the shell as little tubercules, thus giving the surface a minutely pustulated appear- 
ance. When these tubercules are more worn by weathering, or by natural abrasion, or 
by etching with diluted hydrochloric acid, then the central pore of each tubercule is well 
seen in most specimens, and the more these tubercules are worn down towards the inner 
surface of the valves, the larger are the perforations ; and vice versa, the farther they 
are from the inner surface the smaller do they become, until at last they disappear. 
The third point T have satisfied myself about is an interesting one, viz. that the per- 
forations do not encroach upon that area of the valve occupied by the reniform and 
probably the nmscular impressions. All over the other areas of the inner surface of 
the valves they are very distinctly perforated. In my opinion Prof. L. de Koninck is 
therefore wrong if he is under the impression that the perforations pass right through 
the shell of Productus semireticulatus" The enlarged illustrations in Sup., PI. XXXVI, 
figs. 15 — 22, have been drawn by myself from species forwarded to me by Mr. Young, 
and in support of his statements. 
At my request Mr. John Young has also prepared the following Notes on the Per- 
forations observed in the internal Shell-structure of various species of Scottish Carboniferous 
Producti'm his collection : — " 1. Productus gi(/anteus (Sup., PI. XXXVI, fig. 21). The 
inner layers of shell in this species are perforated by two series of punctures ; first, a 
series of large irregular punctures, varying from \\X\ to i-g-th of an inch apart in different 
specimens, and secondly, by a very much smaller and more numerous series of perforations 
that occur amongst the larger punctures all over the shell, except upon the muscular and 
reniform impressions. These smaller perforations on some of the larger shells are some- 
what irregular in their grouping, occurring in numerous irregular little patches ; but on 
some smaller varieties of this species these fine punctures assume a more regular 
character, and arc so small and numerous as only to be well seeii under a low power of 
microscope. 
