22G CRETACEOUS LAMELLIBRAXCHIA. 



./'. heciusfula (Sowerby'), from the Upper Greensand of Blackdowii, closely 

 resembles the small forms of the neocomiensis variety of P. gargitis and does not 

 appear to be specifically distinct. The type consists of an imperfect right valve ; 

 a specimen similar to the type, l)ut with both valves preserved, is in tlie 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 Wollemaun 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-hed and Crackers of Atherfield. 



Considerable differences are seen in the position of the umbones, so that some 

 specimens are much more inequilatei'al 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 roimded or truncate character of the anterior margin, tlie 

 . 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 ai"e 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 arc represented l)v a larger niiml)er of individuals 

 than others. One common form (Plate XXXVI, figs. 3, 4), agrees more or less 

 closely with the type of P. ph'cdfa (Sowerby), and it may perhaps be convenient 

 to refer to this as P. giirgitis var. pUrafa. 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 lunidar depression 

 is distinct and usually lai'ge. 



Another form (Plate XXXV, figs. 9, 13) resembles P. neocomiensis (Leymerie) 

 and may be referred to as P. giirgitis var. neocomiensis. The concentric folds are 



1 Mya Ixvinscula, J. de C. Sowerby, 'Trans. Geol. Soc.,' ser. 2 (1836), vol. iv, pp. 241, 340, 

 pi. xvi, fijr. 6 ; Morris, ' Cat. Brit. Foss.,' ed. 2 (1854), p. 212. Panopn'a hevuiscula, d'Orbignv, ' Prodr. 

 de Pal.,' vol. ii (1850), p. 157; Pictet and Campiche, 'Terr. Cret. Ste. Croix' ('Mater. Pal. Suisse," 

 ser. 4, 1865), p. 70. 



