462 BRITISH PAL-EOZOIC FOSSILS. [ Bracutopopa. 
at about half the length of the visceral disk from the beak, but always increasing in depth on the descending 
front to the margin when present; hinge-line as wide as the shell, forming very small conical ears, projecting 
from the steep sides of the shell, with usually a large spinose ridge between them. Receiving valve with the 
visceral portion gibbous, much arched, crossed by numerous, small, obtuse, slightly undulating, and interrupted 
concentric wrinkles, about twice as far apart as the longitudinal striz, which are not interrupted by them. Strize 
coarse, rounded, nearly of equal size on all parts of the shell, separated by deep, very narrow sulci (except when 
the outer layers are removed, when they about equal the strize in size), dichotomising at irregular distances into 
two equal ones, quickly acquiring their original thickness; several large spines scattered irregularly over the 
surface, and often causing two or more of the striz to join at the point of their insertion. Entering valve 
deeply inserted, flattened at the ears, moderately concave at the sides, and with a mesial furrow corresponding 
with the more or less developed one of the opposite valve; the visceral portion with rather more numerous and 
regular wrinkles than the receiving valve, which it joins at about one half the length of the deflected front, 
leaving a large space for the animal. 
The shell I have called P. costellata in the above volume on the Carboniferous Limestone Fossils of Ireland 
resembles the P. su/cata in form, but is very spinose, and the ridges are remarkable for their irregular ine- 
quality in size, and greater coarseness and prominence; it is probably distinct, but possibly a variety of the 
present species. There are three tolerably distinct varieties of this species, which, however, the examination of 
a couple of hundred of good specimens shews, pass insensibly into each other in various ways: these varieties 
are : 
Ist var. P. longispina.—In which there is scarcely any mesial furrow, the size is small, the concentric 
wrinkles on the visceral portion imperfectly marked, the strize slightly smoother and finer than usual. Its average 
width is seven lines, proportional length of visceral disk =, of deflected front ;5,, depth between the valves ;; 
about nine or ten striz in the space of two lines near the front margin, the scattered spines seven or eight lines 
long, considerably thicker than the strie from which they originate. 
This variety passes to the P. Flemingi by getting the striz a little coarser, which again passes into the 
next by the most insensible and varied gradations. 
2nd var. P. lobata (Sow.)—The extreme forms of this variety have the striz coarser than the preceding 
(about six in two lines on middle of front), and a very deep, narrow, mesial sulcus from about the middle of 
53 
visceral disk, gradually deepening to the front margin. The average width eight lines, length of visceral disk 55, 
90 
length of front ~, the spines about equalling the thickness of the striz on which they are placed. 
No two specimens of this variety agree in the depth of the sinus, which may be observed gradually to 
vanish in a handful of specimens, passing by this and the varying fineness of the striz into the preceding 
variety: when the specimens acquire a greater size, the two next varieties are observed to be produced in the 
most gradual manner. 
3rd. var. P. sulcata (Sow.)—In this the average width is one inch seven lines, proportional length of 
visceral disk {%,, length of front °°; about six strize in two lines on middle of front, depth between the valves =. 
In this variety there is generally a shallow mesial furrow from the middle of the visceral disk to the front 
margin, the strize and general surface are even and regular, and there are few or no spines, except a semicircular 
row of very large ones on a rugged ridge between the ears and the steep sides of the shell. This variety is rare, 
and passes insensibly into the next, which is the most common of them all. 
4th var. P. pugilis (Phill.)—Average width two inches, length of visceral disk ;7;, length of front =, depth 
between the valves &. Strize averaging six in two lines on middle of front. This variety is remarkable for 
having the thick spine-bases scattered over the deflected front, about equal to two or rather more of the strize in 
width, and raising a bundle of four or five strize into a longitudinal ridge from one to two lines wide, extending 
from its base to the front margin, giving to well-marked specimens a strongly fluted character. This is the 
well developed adult, and, by the diminution of size, less distinct fluting, smaller spines, &c. may be gradually 
seen to pass into all the preceding varieties: the spines on the semicircular ridge at the base of the ears are 
upwards of two inches long. For difference from P. semireticulata, see that species. 
Position and Locality.—All the varieties common in the carboniferous limestone of Derbyshire; carboni- 
