70 CRETACEOUS LAMELLIBRANCHIA. 
generally evenly rounded, sometimes forming an obtuse but rounded angle with 
the hinge-margin. Posterior extremity slghtly angular; dorsal part of the 
posterior margin subtruncate, straight or slightly curved. Hinge-line long. 
Umbones prominent, pointed; valves compressed in the postero-dorsal region. 
Hinge-area broad, with several ligament-grooves. Ornamentation consists of broad 
radial ribs bearing finer ribs, and separated by narrow grooves ; the ribs are crossed 
by lines of growth and by faintly marked concentric ridges." On the anterior and 
posterior parts of the valves the radial ribs are less distinct. Central teeth (two 
to four) small, transverse ; lateral teeth (eight to ten) long and oblique. Anterior 
adductor impression large, subtrigonal; posterior adductor a little smaller and 
rounded. Margins of valves strongly crenulate. 
Measurements : 
(4) Average of 
(1) (2) (3) 27 specimens. 
Length . 18 ; 17 : 15 : 18°851 mm. 
Height . 20 : 19 ‘ 16 : 19:924 ,, 
(1—4) from Blackdown. 
Afjiivities—This species differs from P. sublevis, Sowerby, in the shell being 
less stout, more inflated, and usually higher than long; also in having a broader 
hinge-area, more prominent umbones, and fewer ribs. It resembles P. woricus, 
Zittel, in general form, but that species appears to be usually larger, with rather 
less prominent umbones, fewer ribs, and less oblique lateral teeth. 
2, which was referred 
by Goldfuss (vide supra) to this species, has been regarded as distinct by Rémer,’ 
The form from the Senonian of Coesfeld and Quedlinbur 
and named P. decussatus; a fuller description, but without figures, is given by 
Brauns.* I have seen no examples of that form. The name was changed by 
d’Orbigny ° to subdecussatus, since decussatus had been used previously (1813) by 
Sowerby for an Hocene species. 
Types.—From Blackdown; in the British Museum. 
Distribution.—Blackdown Greensand (zone vii, and occasionally beneath it). 
Lower Gault (zones vi and vii) of Folkestone (fide Price). Folkestone Beds of 
Copt Point, Folkestone.° 
1 Seen in only a few specimens. 
2 “ Die Bivaly. der Gosaugeb.,” pt. i, ‘ Denkschr. der k. Akad. der Wissensch. Math.-nat. Cl.,’ vol. 
axiv (1865), p. 167, pl. ix, fig. 9. 
3 *Die Verstein. des norddeutsch. Kreidegeb.’ (1841), p. 69. 
4 «Die senon. Mergel des Salzberges, &c.,” ‘ Zeitsch. f. d. gesammt. Naturwiss.,’ vol. xlvi (1876), 
p- 383. 
5 *Prodr. de Pal.,’ vol. ii (1850), p. 243. 
® There are three specimens from this horizon in the Woodwardian Museum; the surface of the 
shell is not perfectly preserved, but I have very little doubt that they are referable to this species. 
