20 CRETACEOUS LAMELLIBRANCHIA. 



G and 7 are Gardner's figured specimens of N. omatissima and N. bivirgata 

 respectively. 



Affinities. — The forms in which the ribs diverge from two lines (thus AA) were 

 regarded by d'Orbigny as constituting a distinct species — iV. omatissima ; 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. Detval qnei, Briart and Cornet, 1 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. 



JV". picturata, Yokoyama, 2 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. — I 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 vii) of Folkestone ; also, perhaps, 

 Upper Gault, according to Gardner (?). Gault of Black Ven. Cambridge 

 Greensand. 



1 "Descri|)t. Min., Geo]., et Paleout. de la Meule de Bracquegnies" (' Mein. couron. et Mem. 

 des Sav. etraug. Acad. Roy. Belg.,' vol. xxiv, 1868), p. 62, pi. v, figs. 26 — 28. 



2 " Verstein. aus der japaniscb. Kreide," ' Palseontographica,' vol. xxxvi (1889), p. 18, pi. xxv, 

 figs. 1, 2. 



