SONNINIA CRASSINUDA. 351 
The side view of an adult specimen reduced to two-thirds of natural size is shown 
in Pl. LX XXII, fig. 1, and the outline of its whorl-section, of natural size, in fig. 2. 
Sonninta oRassinupDA, S. Buckman. Plate LXXXI, figs. 1—4. 
Discoidal, compressed, carinate. Whorls, in section, oblong, ornamented only 
with a few irregularly-placed obscure ribs and bulges (the spinous and costate 
stages confined to the inner whorls). Ventral area arched, divided by a small 
doubtfully-hollow carina. Inner margin well-marked, broad, upright, nearly flat. 
Inclusion about one-half in immature shells, rather less in older specimens. 
Umbilicus deep, slightly excentric, ornamented with spines and small coste. 
I have included three forms in this species : 
a. Vhick (Pl. LXXXT, figs.1, 2). 
B. The type-form. ‘Thinner, more costate, but less umbilicate (Pl. LXXXI, 
figs. 3, 4). 
y. Like B, but smoother. 
Possibly the association of a and 8 in the same species is incorrect. Except, 
however, that a is smoother than # in the last part of the whorl, it seems to 
exactly link crassa with 8; while y is obviously a descendant of 8, exhibiting 
smoothness at an earlier age. Practically, therefore, it may be supposed that the 
forms of this species show three developmental stages from crassa retrograding 
towards a smooth form. 
From crassa, Sonn. crassinuda may be distinguished by the reduction of 
ornament in regard to the spinous and costate stages, and by the practically 
smooth whorls of the adults. From most of its morphological equivalents it may 
be known by its upright ribs; but it is therefore like dominans. It is, however, 
decidedly more spinous, and spinous to a later date; the succeeding costate stage 
is more irregular and less developed, the ribs’ being smaller and closer together ; 
while smoothness is a feature of adolescence. There is thus much more contrast 
between the ornamentation of the inner and outer whorls than in dominans. 
I have only figured young forms of this species. Adults (160—180 mm. in 
diameter) show whorls practically smooth except for some obscure distant 
bulgings. They also exhibit a tendency for the umbilicus to become excentric, 
and in correlation therewith a slight increase in the proportionate thickness of 
the whorl. 
1 Tn the type-form the ribs are very irregular, many of the smaller ribs only developed on the outer 
two-thirds, and not on the inner area. The ribbing is somewhat like that shown for fig. 5 (Pl. LXXX]J) ; 
but the larger ribs are more distant, and are parted by three or four short-length smaller ribs. 
