SONNINIA CAMURA. 403 



ribbing, a rather different whorl-section, stronger and more persistent spinosity, 

 and rather less costation ; from the second by greater spinosity, by a larger 

 umbilicus, and by the coarser costation, as well as by septation. 



Although the tetradactyloid character of L and its isosceloid terminal lobule 

 are similar to the septal features of crassiformis, and suggest the present position 

 for this species, yet the entirely different proportions and degree of ornament 

 make it doubtful if the forms are closely connected genetically. It may be 

 observed that the outer lateral lobule of L is hardly cleft to the base as in 

 crassiformis. 



Sonn. dwplicata occurs in the Goncaviim-zone of Bradford Abbas. The side 

 view of a specimen in which the spines have been saved in a fairly perfect 

 state is shown in PI. XCIX, fig. 1 ; its whorl-section in fig. 2 ; and its suture- 

 line in fig. 3. 



Sonninia camura, 8. Buchman. Plate XCIX, figs. 4 — 6. 



Discoidal, compressed, carinate. Whorls ornamented with small, closely-set, 

 direct, ventrally-inclined costae, which become larger and more distant after about 

 170 mm. diameter, until in old age they are large, somewhat distant, coarse folds. 

 (The spinous stage ends at about 22 mm. diameter.) Ventral area rounded, 

 divided by a small, inconspicuous, ill-defined carina. Inner margin well marked, 

 broad, nearly upright, rather flat. Inclusion of half-grown shell two-fifths, 

 diminishing in old age to one-fourth. Umbilicus very excentric — a small centre 

 with a rapid and not regular increase. Suture-line 1 with a tetradactyloid L, the 

 outer lateral lobule being in duplicate — cleft nearly to the base, — the terminal 

 lobule slightly axial, isosceloid, equicellate. 



Though the ribs are small and closely set, they are more conspicuous than 

 those of modesta, and even in half-grown shells the difference is very marked ; 

 the umbilicus is also more excentric, the carina much less conspicuous. From 

 revirescens this form differs especially in having a much more rudimentary spinous 

 stage, a more excentric umbilicus, flatter whorls, and less marked costation. 



The septal details are the reason for placing this species with Sonn. duplicata. 

 Whether its ancestor had paired ribs is uncertain, the species is too far advanced 

 in the phylogerontic stage to retain any evidence of such a phyletic phase. The 

 irregular excentricity of the umbilicus which makes the end of the whorl almost 

 hooked, and gives the shell a " scaphitoid " appearance, suggests the name 



1 From another specimen. 



