178 
sometimes divided by a mesial furrow corresponding to tliat of the ventral 
furrow. The sides or the dorsal ridge are not always well marked, hut 
frequently merge insensibly into the adjacent parts of the valve. The base 
is always very sinuous in adults, even when they are only of moderate size. 
The surface of both valves is generally smooth, or merely furnished with fine 
concentric lines of growth ; and some specimens, on their lateral regions, 
show traces of radiating ribs. The test is rather thin and perforated, as is 
easily seen on examining some of the specimens from Australia and Belgium. 
In this case the whole surface is covered with well-marked punctures arranged 
quincunxially. Although S. glaher is one of the commonest species of our 
upper Carboniferous Limestone, I have never yet been able to procure in 
Belgium an internal cast, or isolated valves of it, which would enable me to 
study the internal structure or framework. But these are very plentiful 
among the Australian specimens, and so I have satisfied myself that the 
cardinal process is extremely well developed, and very solid. It is supported 
in the ventral valve by two strong diverging dental plates, placed on each 
side of the deltoidal fissure, and prolonging themselves like ridges, not well- 
marked, on each side of the muscular impressions, and coming together again 
in a sharp point near the middle of the length of the valve, or a little below 
it. The impressions of the cardinal muscles form a very elongated oval, 
longitudinally striated, and parted in the middle by a faintly marked double 
septum, limiting the very narrow impression of the cardinal (?) muscle. On 
the sides are found well-developed ovarian spaces (PI. 12, Pig. 1Z»). The 
muscular impressions of the dorsal valve are more or less long, according to 
the specimen, and consist of two impressions of the adductor muscles, pro- 
ducing by their union an elongated oval, divided lengthways by a small 
median septum, at the uj)per extremity of which is a small triangular 
impression striated lengthways, which has probably served as the attachment 
of the peduncle (PL 12, Pig. Ic). The spiral supports are well developed, 
and fill the greater part of the interior of the shell. 
Dimensions . — These are very variable. The following measurements 
are taken from three rather large specimens : — 
Length. 
Breadth. 
Thickness. 
70 
• • • • • • 
76 
40 millimetres. 
55 
95 
36 
67 
80 
57 
I agree with Mr. Dana in considering the specimens described and figured 
(PI. 16 in Strzelecki’s work) by Mr. Morris under the name S. suhradiatus 
