70 CRETACEOUS LAMELLIBRANCHIA. 



generally evenly rounded, sometimes forming an obtuse but rounded angle with 

 the hinge-margin. Posterior extremity slightly 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. 1 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. 



Affinities. — This species differs from P. sublsevis, 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. noricus, 

 Zittel, 2 in general form, but that species appears to be usually larger, with rather 

 less prominent umbones, fewer ribs, and less oblicme lateral teeth. 



The form from the Senonian of Coesfeld and Quedlinburg, which was referred 

 by Goldfuss (vide supra) to this species, has been regarded as distinct by Eomer, 3 

 and named P. decussatus; a fuller description, but without figures, is given by 

 Brauns. 4 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 Eocene 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. 6 



1 Seen in only a few specimens. 



- " Die Bivalv. der Gosaugeb.," pt. i, 'Denkschr. der k. Akad. der Wissenseh. Math.-nat. CI.,' vol. 

 xxiv (1865), p. 107, pi. ix, fig. 9. 



3 'Die Verstein. des norddeutscb. Kreidegeb.' (1841), p. 69. 



4 "Die senon. Mergel des Salzberges, &c," 'Zeitsch. f. d. gesammt. Natunviss.,' vol. xlvi (1876), 

 p. 383. 



5 'Prodr. de Pal.,' vol. ii (1850), p. 243. 



Tbere are three specimens from this horizon in the AVoodwardian Museum ; the surface of the 

 shell is not perfectly preserved, but I have very little doubt that they are referable to this species. 



