138 CRETACEOUS LAMELLIBRANCHIA. 



lamellae and well-marked growth-lines are present. The ribs vary in number 

 considerably in different specimens, but are always more numerous near the ventral 

 margin than near the umbo, owing to the intercalation of new ribs. Left valve 

 ornamented with similar but usually broader and more rounded ribs, crossed by 

 numerous concentric lamellge. 



Measurements : 

 Length 



(1) 

 53 



48 



(3) 



47 



(4) 



39 



(5) 



36 



(6) 



33 



(7) 



31 



(8) 



30 



(9) 



25 



(10) 



22 



(11) 



22 



(12) 



17 mm. 



Height 1 . 



48 



44 



46 



51 



40 



38 



28 



36 



28 



30 



29 



20 „ 



Number of ril3S 



























at margin of 



























riofht valve . 



39 



34 



25 



29 



23 



19 



19 



24 



24 



26 



28 



11 



(1 — 7, 11, 12) from the Grault of Folkestone; (8 — 10) from the Gault near Cambridge. 



Affinities.— ^ee F. infiata (p. 140). 



Bemarhs. — This species was described by Sowerby as Plicatula pectinoides 

 (Lamarck). The type of Lamarck's species came from the Lias of Metz," and is 

 quite distinct from the Cretaceous species ; the latter must, therefore, be known as 

 P. (jur gifts, Pictet and Roux. 



Types. — The types of P. pectinoides, Sowerby, from the Gault near Cambridge, 

 appear to have been lost. 



Distribution. — G-ault of Folkestone (zones i — iii, vi, viii, x, xi), of Ford (near 

 Aylesbury), and of Barnwell (Cambridge). Cambridge Greensand (derived from 

 the Gault). Red Limestone of Hunstanton. Upper Greensand of the Isle of 

 Wight. 



Plicatula minuta, Seelei/, 1866. Plate XXV, figs. 22—25. 



1866. Plicatula minuta, H. G. Seelcij. Auu. Mag. Nat. Hist., ser. 3, vol. xvii. 



p. 176. 



liemarJcs. — The ornamentation of this small form is of the same type as that of 

 P. gurgitis, but the ribs are perhaps more numerous than at the umbo of that 

 species. Comparison, however, is difficult, since the umbo of P. gurgitis is seldom 

 well preserved. It seems probable that P. minuta is only the young form of P. 

 gurgitis, but since the smallest known example of the latter is very much larger 



1 Measured obliquely from the umbo to the middle of the ventral margin. 



■^ Placuna 2^ectinoides, Lamarck, ' Anim. sans Vert.,' vol. vi (1819), p. 224. Plicatiila pectinoides^ 

 Defrance, 'Diet. Sciences nat.,' vol. xli (1826), p. 400; Dcshayes and Milne Edwards, Lamarck's 

 ' Anim. sans Vert.,' ed. 2, vol. vii (1836), p. 178; d'Orbigny, 'Prodr. de Pal.,' vol. i (1849), p. 238. 



