90 PERMIAN FOSSILS. 



lobes concave, in those found at Humbleton (PL XI, fig. 11). I do not insist on this 

 being a positive difference, because there is some appearance cf 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, PI. 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 (PI. XI, fig. 9), in which these spines were very distinct, and hollow like 

 the others. The abortive spines 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, PI. 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 palaeontologist) 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, in 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 

 in 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, PI. XI: 



