







PECTEN. 











Mciii^nremenix : 



















(1) (2) 



(3) 



(4) (■>) 



CO 



(7) ("8) (0) (10) 



(11) 



(12) (13) 



(It.) 



(15) (1(5) (17) 



Length . 43 41 



37 



33 20 



33 



5G GO 50 40 



38 



28-5 13 



36 



50 60 35 



Height . 53 51 



47 



41-5 25 



44 



65 73 06 50 



49 



34 17 



47 



58 74 45 



173 



(IS) 



(1-5) Gault, Folkestone. \ (!> -13) Grey Chalk, Dover. 



(6) Upper Greoiisand, Warminster. (14) Chalk Marl, Ventnor. 



(7) „ „ Veutuor. i (15— 17) Lower Chalk, Burwell. 



(8) Malinstoue, Wilton. l (18) Tottenihoe Sloue, Arlesey. 



Affinilh-fi.—^YhQ examples from the Gaiilt (usually named P. Iidiiliiilfiiiiis) have 

 been regarded as distinct from those found in the Upper Greensand and C'halk (/'. 

 pJoiir/afit.^) ; 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 Avell-marked as in the latter. The first difference is, I think, readily 

 explained by the less j)erfect preservation of the spiny scales in specimens from the 

 pervious beds of the Upper Greensand and Chalk ; tlie triple arrangement of the 

 ril)s varies coiisideral)lv in different specimens of 1'. cloiHjafns 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. Mr. Jukes-Browne and Dr. F. L. 

 Kitchin have examined carefully a number of specimens, and agree with me in 

 considering that the differences between P. eloiigatns and 1\ Rauiiniaiius are not of 

 specific value. 



/'. Miirrotiantia, d'Orbigny,^ from the Upper Senonian of Dordogne, resembles 

 r. clniu/atK.^, Ijut is apparently distinguished by the ears being 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. Fcriijasi, Defrance,^ from the P. mucronata Chalk of Maestricht, approaches 

 very closely those forms of P. ehmgatns which have the triple arrangement of the 

 ribs well developed, l^ut appears to differ in having fewer ribs. 



BpmnrJcs. — The specimens figured as i'. L'nuliniaiins by Pictet and Roux, and 

 by Pictet and Campiche, seem to differ from the English forms in haN'ing k^wcv 

 ribs ; in this respect, however, the figm-es of those Avriters do not agree Avitli theii- 

 descriptions. But since Pictet and Campiche obtained specimens from Folkestone 

 for comparison it is probable that their determination is correct. The example 



1 ' Pal. Franc. Terr. Cix't.,' vol. iii (1847), p. 612, pi. ccccxxxviii, figs. 1 — 6. 



" Faiijas-St.-Foua, ' Hist. Nat de la Mont, de St. Pierre de Maestricht' (1799), p. 153, pi. xxiv, 

 fig. 5; Defrance, 'Diet. Sei. nat.,' vol. xxxviii (1825), p. 265; Goldfuss, ' Petref . Germ.,' vol. li 

 (1833), p. 57, pi. xciii, fig 7 ; > Reuss, ' Versteiii. bohm. Kreide format.,' pt 2 (1846), p. 30 ; SchrGder, 

 ' Ze'tsehr. d. deutscli. geol. Gesellsch.,' vol. xxxiv (1882), p. 265 ; Vogel, ' Verhandl. nat. Vereius d 

 preussisch. Rheinl.,' vol. xlix (1892), p. 59, and ' Hollandiseh. Kreide' (1895), p. 24, pi. i, fig 22. 



