FOEAMINIFERA OF THE WEST OF SCOTLAND. 279 



Family NUMMULINIDvE. 



Subfamily Poitstomeilinj!. 



Nonionina, d'Orbigny. 



308. Noilionina depressula (Walker & Jacob). (Plate 43. figs. 4-7.) 



Nautilus depressulus, Walker & Jacob, 1798, AEM. p. 641, pi. xiv. fig. 33. 

 Nonionina depressula, Brad)', 1884, FC. p. 725, pi. cix. figs. 6, 7. 



23 Stations. 



Almost universally distributed, abundant at many Stns., and, as usual, subject to 

 excessive variation, due primarily to tbe cbaracter of the sutural lines, which are some- 

 times depressed and sometimes litnbate. The best examples of typical N. depressula 

 occur at Stns. 5 and 22. 



Among the most noticeable and constant varieties is one which we figure, and which 

 forms the principal feature of Stn. 17, characterized by deeply excised sutural lines ; in 

 some cases the marginal edge is scolloped as a result of this feature. The umbilical 

 portion of the shell is filled in, sometimes a stud of solid shell-matter appearing in 

 the centre. This appears to be the Nautilus spiralis of Walker and Boys (W. & B. 

 1784, TM. p. 19, pi. iii. fig. 68). It occurs in smaller numbers at many other Stns. 

 Another widely distributed variety is characterized by a biconvex hyaline and strongly 

 punctate shell with limbate sutures. This is the form to which we have referred in our 

 Kerimba Mongraph (H.-A. & E. 1914, etc., FKA. 1915, p. 730). Monstrous specimens 

 due to fusion of two and sometimes more individuals occur at Stns. 5 and 17. 



309. Nonionina umbilicatula (Montagu). 



Nautilus umbilicatulus , Montagu, 1803-8, TB. p. 191, Suppl. p. 78, pi. xviii. fig. 1. 

 Nonionina umbilicatula, Brady, 1884, FC. p. 726, pi. cix. figs. 8, 9. 



20 Stations. 



Almost universally distributed and often very common. There is a certain amount 

 of variation due (i.) to the degree of turgidity in the growth of shell, and (ii.) to the 

 degree of envelopment of the chambers of the final whorl. In young specimens there is 

 no depression at the umbilicus. With increase in size, if the shell continues of the non- 

 turgid type, the whorls are almost entirely embracing, so that the umbibcal region 

 remains either almost flush or very slightly depressed. If the chambers are of the 

 turgid type approaching N. pomjnlioides (F. & M.) each successive convolution becomes 

 less enveloping, so that the umbilicus becomes deep; and at some Stns. the successive 

 later whorls are undercut, exhibiting two entire convolutions in the centre of the shell. 

 Distjited specimens occur at Stn. 18. 



42* 



