90 PERMIAN FOSSILS. 
lobes concave, in those found at Humbleton (PI. XI, fig. 11). Ido not insist on this 
being a positive difference, because there is some appearance of its being due to 
dissimilar modes of fossilization. 
This species is remarkable for its small valve possessing two sets of spines, as 
exhibited on the casts from Humbleton, represented by figs. 6,7, Pl. XI. One set con- 
sists of one or more rows of perfect spines confined to each lateral half of the cardinal 
region, and slightly divaricating radiately from the hinge-line as they strike off from its 
centre: they are hollow, and have evidently had a permanent communication with the 
inner surface of the valve. The Derbyshire specimen, represented by fig. 2, PI. XI, beauti- 
fully exhibits these spines; and I have seen them nearly as prominently displayed in a 
specimen collected at Garmundsway. ‘The other set consists of abortive spines, that 
is, they have never been developed to the extent of the latter; nor do they communi- 
cate with the inner surface of the valve, at least when the shell is in an advanced 
stage of growth: it is necessary to make this reservation, as I have seen young 
specimens (Pl. XI, fig. 9), in which these spines were very distinct, and hollow lke 
the others. The abortive spmes have a somewhat regular radiate arrangement. The 
spines of the opposite valve appear to have always communicated with its inner 
surface: they occasionally display a tendency to follow the same arrangement as those 
on the small valve; and exhibit no other difference among themselves than being more 
crowded in the cardinal rows. The fossils represented by figures 2, 5, Pl. XI, will afford 
a tolerably good idea of the size and other peculiarities of the cardinal spines of both 
valves. It is very seldom that a specimen is found with these appendages preserved ; 
but occasionally the investing matrix shows them attached to the surface of the large 
valve; when their direction and: length may be pretty accurately determined. I 
have seen a spine thus attached, apparently complete, measuring full two inches in 
length. 
The Humbleton Hill specimens of this shell generally occur in the state of casts, 
which, although on this account are deficient as regard external characters, so finely 
displayed in some of the Derbyshire specimens, are, nevertheless, highly instructive as 
respect the internal structures. Having availed myself pretty freely of gutta percha 
(so indispensable to a paleontologist) in the way of taking impressions of these casts, 
I have been enabled to reproduce, as it were, the entire inner surface of a number of 
small valves, with all their respective structures as perfectly displayed as when the shells 
to which they belong were inhabited by their living occupants. One of these gutta- 
percha casts is represented by figure 10, im Plate XI: all the structures thereon 
displayed are more or less seen on impressions I have taken from a number of 
specimens; but in this one their combination and sharpness are better exhibited than 
im any of the others. In noticing these structures, attention must be directed, in the first 
place, to the boss or cardinal muscular fulcrum (a), which has precisely the same bilobed 
form already noticed, as occurring in the specimen represented by fig. 11, Pl. XI: 
