298 
SUPPLEMENT TO THE BRITISH 
the inner surface of the valves, as raised tubercules. The spines in these rows are 
numerous, about one half line apart, and on both valves are seen to have been 
reflexed or bent backv\^ards to the hinge portion of the shell. I -have also to note these 
marginal rows of spines as occurring in several of the other species, 
" 4. Prochdus costatus (Sup., PI. XXXVI, figs. 18, 19, 20).— Nearly all my examples 
of this species are out of soft shale, and therefore have the surface of their shell wqW 
preserved, and show no perforations. To show these in the inner layer I have 
etched the outer surface with acid, and as the outer shell was gradually removed the 
perforations began to make their appearance on the inner layer, and by the time the 
etching was completed they were found to be quite as numerous, and of the same 
character, as those perforations seen in Prod, semireticulatus. When the whole shell has 
been entirely removed by acid, we have a beautiful cast of the perforations on the sub- 
stance filling the interior of the shell in the form of a series of small raised tubercules. 
The perforations do not occur upon either the muscular or reniforin impressions, and they 
apparently terminate, at least in the dorsal valve, before they reach the outer border of 
the shell. A finely preserved interior of the dorsal valve shows that the outer border 
is adorned with a numerous series of small raised tubercules, which probably are due to the 
continuation inwards from the surface of the shell of small hair-like spines. Between this 
outer tuberculated border and the inner perforated surface of the valve, there is a narrow, 
smooth line, running round the shell and separating the two, and upon this line there 
is no perforations nor tubercules, a character which I have not yet observed in any 
other species. 
" 5. Productus punctatus. — In this species the numerous small tubular spines of vary- 
ing size are continued into the interior surface of the valves, where they appear as raised 
tubercules, of which, on many specimens, those belonging to the larger spines are best 
seen. When these internal tubercules are slightly worn down on their surface they reveal the 
tubular character of the spines passing outwards to the surface of the shell. These hair- 
like spines seem in some instances to have been an inch or more in length, as specimens 
in my collection show them three quarters of an inch long, and imperfect or broken short 
at both ends. Only in one fairly preserved interior of a dorsal valve from Campsie, and 
on a portion of another interior from Beith, Ayrshire, have I as yet been able to discover 
perforations passing outwards through the inner layer of the shell. These perforations 
can be distinctly seen with a pocket lens. They are very numerous and are seen to be 
somewhat irregular in their arrangement amongst the spine tubercules. 
" 0. Productus longispinus (Sup., PI. XXXVI, figs. 14, 15, 16).' — The best examples of 
this species that I have obtained, showing the perforations, are from the Blantyre Limestone 
Shales. These, when treated with acid, show the perforations to be numerous on the 
upper portion of the ventral valves, and quite visible with a pocket lens ; but on the 
level of the two frontal spines, that are so characteristic of the species, a curious change 
takes place in the character of the perforations, so distinctly seen on the upper portion of 
