GS 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 : 













Length . 



(i) 

 24 . 



(2) 



22 



(3) 



. 16 



(4) 



. 13-5 



(5) Average of 

 29 specimens. 



. 18-836 mm, 



Height . 



22-5 . 



21 . 



16 . 



13 



. 18-077 „ 



(1 — 5) from Blackdown. 

 Affinities. — A species from the Aachen Greensand was described and 

 figured by Goldfuss as P. sublsevis, Sowerby ; but it has been shown by later 

 authors to be distinct from the latter; Romer 1 and also Brauns, 2 have referred it 

 to P. lens, Nilsson, 3 but the characters of that form are known only from 

 internal casts. Bohm 4 has named the Aachen form P. dux, but Holzapfel, 5 in a 

 later work, refers it to P. Geinitzi, d'Orbigny, 6 the type of which is P. sublsevis, 

 Geinitz, 7 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. sublsevis, Sowerby, in 

 the following features : 



(1) The average size of the adults is considerably larger, and the valves 

 are less inflated than in P. sublsevis. 



(2) The hinge-area is smaller and the external teeth shorter. 



(3) 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 strias. 



1 ' Die Verstein. des norddeutsch. Kreidegeb.' (1811), p. 68. 



2 "Die senou Mergel des Salzberges., &c," 'Zeitscb. f. d. gesammt. Naturwiss.,' vol. xlvi (1876), 

 p. 383. 



3 'Petrif. Suecana' (1827), p. 15, pi. v, fig. 4; ITennig, ' Ilevis. af Lamellibr. i Nilsson's "Petrif- 

 Suecana," ' Kongl. Fysiogr. i Lund Hand].,' vol. viii (1897). p. 63. 



1 ' Verhandl. des naturbist. Vereinea der preuss. Eheiul., &c.,' vol. xlii (1885), p. 93. 

 "Die Mollusk. der Aashen. Kreide.," ' PalsBontographica,' vol. xxxv (1889), p. 210, pi. xxiii, 

 Gg3. M. L2; pi. xxiv, figs. 1—10. 



fi ' Prodr. de Pal.,' vol. ii (1S50), p. 196. 



7 ' Die Verstein. von Cieslingswalda ' (1813), p. 14, pi. ii, figs. 19—21. 



