SONNINTA SUBSIMPLEX. 427 



Sonninia sdbsimplex, S. Buckman. Plate XCV, figs. 6 — 8 ; and variety (?), 



Plate CM, fig. 19, suture-line. 



1892. Sonninia subsimplex. This Monograph, p. 328. 



Discoidal, compressed, subcarinate. Whorls nearly smooth, showing only some 

 small, closely-set, obscure, direct costse, which are coarser, more distant, and often 

 liable to local failure, and never very prominent in adult. Ventral area subacutely 

 arched, subcarinate — the carina very small and ill-defined ; after about 160 mm. 

 diameter, flattened, and the carina a little more marked, subimpressed. Inner 

 margin fairly defined, steeply sloped, barely subconvex. Inclusion about one- 

 quarter. Umbilicus markedly excentric. Whorl-section oblong. Suture-line 

 with a very intra-axial, anisosceloid, terminal lobule to L; the lateral lobules 

 of L anisometric, the outer tripartite, the inner slightly bipartite ; the inferior 

 lateral lobe similar to L, but its terminal lobule even more intra-axial, the two 

 principal lateral lobules well developed. 



The above description does not quite apply to the young form ; that is slightly 

 more carinate and more involute : at a diameter of about 135 mm., when the 

 excentricity of the umbilicus first begins to be noticeable, the inclusion is nearly 

 one-half. 



The septal margin shown in PI. CM, fig. 19, belongs to what may be a muta- 

 tion of S. sub simplex. In proportion the shell is similar, though its umbilicus is 

 more excentric on account of less umbilicate central whorls ; the test is smoother, 

 the septa are decidedly more ornate, and different in many respects from those of 

 Sonn. subsimphx — for instance, the siphonal saddle is more occupied by lobes, 

 the terminal lobule of L is more anisosceloid, and L is more symmetrical. The 

 chief interest that attaches to this specimen is that it is, in side view, practically 

 indistinguishable from Sonn. simplex ; in front view it is a mere trifle thinner, but 

 in septation (compare figs. 19 and 11 of PI. CM) there is a most astonishing and 

 remarkable difference. 



Sonn. sub simplex (type) is similar to modesta, but thinner ; the ventral area is 

 more fastigate ; the carina is smaller and less defined ; the ornamentation is 

 similar, but the ribs are less conspicuous ; the umbilicus is more excentric, the 

 suture-line is shorter in its lobes, and the saddles are less occupied ; the inner 

 lobule of L is much less abbreviated ; the terminal lobule is more intra-axial. 

 From Sonn. simplex this species differs by being carinate throughout life, by its 

 umbilicus being more concentric, especially in the early whorls, by not being so 

 smooth, and by having more florid septa. 



