CARBONIFEROUS BRACHIOPODA. 
289 
better and larger examples have been found in Scotland by Mr. James Thomson ; and 
this has induced me to add some illustrations to those already published, so as to show 
what were the dimensions the shell acquired in certain favoured localities. Mr. Thomson 
has discovered at Auchenskeigh, Dairy, in Ayrshire, bands of limestone and shales 
literally covered by, or nearly made up of flattened individuals, of all sizes and ages, 
lying one over the other in such profusion that the sea-bottom on which they lived 
must have been thickly strewed with innumerable individuals of this species. The 
specimens measure from two lines to six inches in breadth, by one line to three inches 
and a half in length. 
In these fine slabs at Auchenskeith and Broadstone, Beith, the shells are generally, 
as already stated, much flattened by pressure ; many are quite perfect, while the greater 
number are in a more or less fragmentary condition ; single valves abound, and some show 
the interior surface in a very perfect manner. In all the external sculpture is admirably 
preserved, and as sharp as if the shell had just been taken from the sea-bottom. They 
must have lived on the spot where they are now found, or near it, on account of the 
sharpness of their exterior sculpture, and could not have floated or been driven by the 
action of the waves for any great distance, or their surface would have been subjected to 
abrasion, of which no evidence is observable. 
Mr. J. Young informs me that he has discovered that Streptorhynchus crenistria is 
perforated from the inner surface very much in the same manner as Productus ; and, as in 
it, the perforations do not reach the outer surface of the shell. 
I feel still disposed to adhere, in the main, to the views expressed at p. 134 of my 
Monograph with respect to this most variable species ; and, indeed, since I have had the 
advantage of examining a very great number of specimens, I would hardly know where 
to draw a distinctive line between the typical, very flat specimens of Strept. crenistria, 
and the very convex var. senilis of Phillips, although I am of opinion that the varietal 
name should be maintained. 
When one can examine a large series of specimens of Strep, crenistria, a great 
difierence is observable in the degree of flatness or convexity of its valves ; also in the size 
or proportion of the area, number and closeness of the numerous radiating, raised striae 
or ribiets that cover the surface of its valves ; in the width also of the interspaces between 
them. In a specimen from Auchenskeith, in the collection of Mr. James Thomson (Sup., 
PI. XXXVII, fig. 2), measuring five inches in breadth, I have counted round the margin 
as many as 250 raised stria;, with interspaces slightly larger, while at half the length of the 
valve not many more than 100 could be numbered, with interspaces three times wider 
than that of the riblet ; indeed, the increase by interpolation in the number of smaller 
ribiets or raised strise existing between the principal ones varies exceedingly. Thus, in 
some examples only one or two smaller ribiets will be found in the interspaces, while in 
other examples as many as eight or nine may be counted. In some specimens the 
principal ribs become gradually stronger as they extend from the beaks to the margin, 
