] 
PECTEN. 17: 
Measurements : 
(Ly (2) (8) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) 
Length . 43 41 37 33 20 35 56 60 56 40 38 285 13 36 50 60 35 55 mm. 
Height .53 51 47 41:5 25 44 65 73 66 50 49 34 17 47 58 74 45 65 
(1—5) Gault, Folkestone. (9-13) Grey Chalk, Dover. 
(6) Upper Greensand, Warminster. (14) Chalk Marl, Ventnor. 
(7) s Ventnor. (15—17) Lower Chalk, Burwell. 
(8) Malmstone, Wilton. | (18) Totternhoe Stone, Arlesey. 
Affinities. —The examples from the Gault (usually named P. Raulinianus) have 
been regarded as distinct from those found in the Upper Greensand and Chalk (21’. 
elongatus); in the former the ribs usually appear to be more prominent owing to 
the longer and more pointed scales, and generally the triple arrangement of the 
ribs is not so well-marked as in the latter. The first difference is, I think, readily 
explained by the less perfect preservation of the spiny scales im specimens from the 
pervious beds of the Upper Greensand and Chalk ; the triple arrangement of the 
ribs varies considerably in different specimens of P. elougatus from the Chalk and 
Greensand, and some forms with less regular triplets seem to be quite inseparable 
from the examples found in the Gault; on the other hand, a few Gault specimens 
have the triplets well and regularly developed. Ma. Jukes-Browne and Dr. IF. L. 
Kitchin have examined carefully a number of specimens, and agree with me in 
considering that the differences between P. clongatus and P. Raulinianus are not of 
specifie value. 
P. Marrotianus, VOrbigny,' from the Upper Senomian of Dordogne, resembles 
P. elongatus, but is apparently distinguished by the ears bemg less unequal, and by 
the anterior left ear having fewer and more distinct ribs ; also the scales on the ribs 
of the valve are closer together and more numerous, and in the grooves between 
the triplets of ribs there are, in the adult, two small ribs. 
P. Faujasi, Defrance, from the 2B. mucronata Chalk of Maestricht, approaches 
very closely those forms of 1. elongatus which have the triple arrangement of the 
ribs well developed, but appears to differ in having fewer ribs. 
Remarks. 
The specimens figured as 2. Raulinianus by Pictet and Roux, and 
by Pictet and Campiche, seem to differ from the English forms in having fewer 
ribs; in this respect, however, the figures of those writers do not agree with their 
descriptions. But since Pictet and Campiche obtained specimens from Folkestone 
for comparison it is probable that their determination is correct. The example 
1 «Pal. Frane. Terr. Crét.,’ vol. iii (1847), p. 612, pl. cecexxxviii, figs. 1—6. 
2 Faujas-St.-Fond, ‘ Hist. Nat. de la Mont. de St. Pierre de Maestricht’ (1799), p. 153, pl. xxiv, 
fig. 5; Defrance, ‘ Dict. Sci. nat.,’ vol. xxxviii (1825), p. 265; Goldfuss, ‘Petref. Germ.,’ vol. 11 
(1833), p. 57, pl. xciii, fig 7; ? Reuss, ‘ Verstein. bohm. Kreideformat.,’ pt. 2 (1846), p. 30; Schroder, 
‘Ze'tschr. d. deutsch. geol. Gesellsch.,’ vol. xxxiv (1882), p. 265; Vogel, ‘ Verhandl. nat. Vereins d 
preussisch. Rheinl.,’ vol. xlix (1892), p. 59, and ‘ Hollandisch. Kreide’ (1895), p. 24, pl. i, fig. 22. 
