68 CRETACEOUS LAMELLIBRANCHIA. 
anterior border; posterior border subtruncate, dorsal part of it nearly straight ; 
posterior extremity slightly angular. Umbones of moderate size. Hinge-line 
long. Hinge-area moderately broad, with many ligament-grooves. Ornamenta- 
tion consists of broad radial ribs separated by narrow grooves—the main ribs 
bearing smaller ribs; this ornamentation is less distinctly marked antero-dorsally 
and postero-dorsally, the change in passing to the former area being gradual, to 
the latter rather sudden. At distant intervals there are lines of growth. Hinge- 
plate stout, with two or three small transverse teeth ; and oblique, moderately long 
lateral teeth (usually six to nine). Adductor impressions well marked; the 
anterior subtrigonal, the posterior smaller and oval. Margins strongly crenu- 
lated. 
Measurements : 
(5) Average of 
: (1) (2) (3) (4) 29 specimens. 
Length . 24 . 22 6s S25) ee e883 Oman: 
Elcie hte ss22cote #2 ee een LON ses eels ar UUSHOUT 
(1—5) from Blackdown. 
Affinities —A species from the Aachen Greensand was described and 
ficured by Goldfuss as P. sublevis, Sowerby; but it has been shown by later 
authors to be distinct from the latter; Romer! and also Brauns,” have referred it 
to P. lens, Nilsson,* but the characters of that form are knowu only from 
internal casts. Bohm* has named the Aachen form P. dux, but Holzapfel,’ in a 
later work, refers it to P. Geinitzi, d’Orbigny,° the type of which is P. sublexvis, 
Geinitz,’ non Sowerby, from Kieslingswald. The Aachen species is rather 
variable, but a comparison of several specimens and of the good series of figures 
given by Holzapfel, shows that it generally differs from P. sublevis, Sowerby, in 
the following features : 
(1) The average size of the adults is considerably larger, and the valves 
are less inflated than in P. sublevis. 
(2) The hinge-area is smaller and the external teeth shorter. 
(5) The shell is more quadrate in form, and, as a rule, a little higher than long. 
(4) The ribs are more flattened and are separated by striz. 
1 *Die Verstein. des norddeutsch. Kreidegeb.’ (1841), p. 68. 
9 NT 
2 “ Die senon Mergel des Salzberges., &e.,”’ ‘ Zeitsch. f. d. gesammt. Naturwiss.,’ vol. xlvi (1876), 
3 « Petrif. Suecana’ (1827), p. 15, pl. v, fig. 4; Hennig, ‘ Revis. af Lamellibr. 1 Nilsson’s “ Petrif: 
Suecana,” ‘ Kongl. Fysiogr. i Lund Hanadl.,’ vol. viii (1897), p. 63. 
* «Verhandl. des naturhist. Vereines der preuss. Rheinl., &e.,’ vol. xlii (1885), p. 93. 
° “Die Mollusk. der Aachen. Kreide.,”’ ‘ Paleontographica,’ vol. xxxv (1889), p. 210, pl. xxiii, 
figs. 11, 12; pl. xxiv, figs. 1—10. 
6 «Prodr. de Pal.,’ vol. ii (1850), p. 196. 
7 «Die Verstein. von Kieslingswalda’ (1843), p. 14, pl. ii, figs. 19—21. 
