20 CRETACEOUS LAMELLIBRANCHIA. 
6 and 7 are Gardner’s figured specimens of N. ornatissima and N. bivirgata 
respectively. 
Affinities —The forms in which the ribs diverge from two lines (thus M) were 
regarded by d’Orbigny as constituting a distinct species—. ornatissima ; the 
only difference being in the form of the ornamentation, and this feature is not 
constant: in some cases the ribs on the earlier part of the shell diverge from one 
line, whereas on the later part they diverge from two lines, or the reverse may 
be the case. The adductor impressions are not so deep in N. bivirgata as they 
are in N., pectinata. 
N. Dewalquei, Briart and Cornet,’ from the Meule de Bracquegnies, is very 
closely related to N. bivirgata, but in the former the line from which the ribs 
diverge is directed more anteriorly than it is in the latter species. N. Dewalquei 
is stated by the writers mentioned to be longer in proportion to its width than 
N. bivirgata; I have seen no examples of the former. 
N. picturata, Yokoyama,’ from the Upper Cretaceous of Poronai (Japan), is 
also allied to N. bivirgata ; I have seen no specimen of that form, and the figures 
do not enable me to make an exact comparison. 
Remarks.—The variation in the proportions of length and breadth is fairly 
considerable. In some cases the line from which the two sets of ribs diverge 
cuts the ventral border of the valve near its middle point, but in others more 
anteriorly, the position varying in different examples. 
Types.—1 have not seen the type; it is apparently lost. The specimens 
figured by Gardner (except figs. 20—22) are in the British Museum; one 
of those figured by Jukes-Browne (fig. 6) is in the Woodwardian Museum, 
Cambridge. 
Distribution.—Lower Gault (zones v and vi) of Folkestone; also, perhaps, 
Upper Gault, according to Gardner (?). Gault of Black Ven. Cambridge 
Greensand. 
1“ Descript. Min., Géol., et Paléont. de la Meule de Bracquegnies”’ (‘Mém. couron. et Mém. 
des Say. étrang. Acad. Roy. Belg.,’ vol. xxiv, 1868), p. 62, pl. v, figs. 26—28. 
2 “WVerstein. aus der japanisch. Kreide,’’ ‘ Paleontographica,’ vol. xxxvi (1889), p. 18, pl. xxv, 
figs. 1, 2. 
