226 CRETACEOUS LAMELLIBRANCHIA. 
P. leviuscula (Sowerby'), from the Upper Greensand of Blackdown, closely 
resembles the small forms of the eocomiensis variety of P. gurgitis and does not 
appear to be specifically distinct. The type consists of an imperfect right valve ; 
a specimen similar to the type, but with both valves preserved, is in the Museum 
of Practical Geology (No. 23,823). 
Remarks.—Previous writers have remarked on the great amount of variation 
shown by this species. At first sight some of the varieties appear to be distinct, 
but, as Wollemann has pointed out, when a large number of specimens are 
studied, transitions can be traced between the different forms. The varieties do 
not appear to be confined to definite horizons, and nearly all of them are found 
together in the Perna-bed and Crackers of Atherfield. 
Considerable differences are seen in the position of the umbones, so that some 
specimens are much more inequilateral than others ; in those in which the anterior 
part is relatively short the anterior slope is rapid. The size and depth of the 
depression in the lunular region show considerable variation; when large and 
deep the umbones are usually narrower and more pointed than when the depression 
is small and shallow, and in the former case the carina extending from the umbo 
to the antero-ventral margin is usually distinct. The relative length and height 
of the shell, the rounded or truncate character of the anterior margin, the 
curvature of the ventral margin, the height of the posterior end, and the convexity 
of the anterior compared with that of the posterior part of the shell also show 
more or less considerable differences. The concentric folds may be conspicuous, 
but are sometimes indistinct; sometimes they appear to be better marked on 
internal casts than on the shell itself; in some cases their prominence is due to the 
state of preservation of the shell; thus Wollemann mentions a specimen in 
which one valve is weathered and shows strong folds, whilst the other valve is 
well preserved and nearly smooth. 
Some forms of this species are represented by a larger number of individuals 
than others. One common form (Plate XXXVI, figs. 3, 4), agrees more or less 
closely with the type of P. plicata (Sowerby), and it may perhaps be convenient 
to refer to this.as P. gurgitis var. plicata. In this the concentric folds are usually 
prominent, the ventral margin is only slightly curved, the posterior part of the 
shell is of nearly the same height as the anterior part, and the lunular depression 
is distinct and usually large. 
Another form (Plate XXXV, figs. 9, 13) resembles P. neocomiensis (Leymerie) 
and may be referred to as P. gurgitis var. neocomiensis. The concentric folds are 
1 Mya leviuseula, J. de C. Sowerby, ‘Trans. Geol. Soe.,’ ser. 2 (1836), vol. iv, pp. 241, 340, 
pl. xvi, fig. 6 ; Morris, ‘Cat. Brit. Foss.,’ ed. 2 (1854), p.212. Panopza leviuscula, d’Orbigny, ‘ Prodr. 
de Pal., vol. ii (1850), p. 157; Pictet and Campiche, ‘Terr. Crét. Ste. Croix’ (‘Matér. Pal. Suisse,’ 
ser. 4, 1865), p. 70. 
