356 INFERIOR OOLITE AMMONITES. 
This species is not common. My specimens are from the Concavwm-zone of 
Bradford Abbas. Side and front views of an immature example are shown in 
figs. 1, 2 of Pl. LX XXIII; and the same views of quite a young form are given in 
figs. 3, 4. It should be noticed that this young form is like the Sonninia, sp., 
figured in Pl. LXXIX, figs. 7, 8; but the spines of its inner whorls are smaller 
and closer together, and its whorl is more compressed. 
Sonninra tavicaTa, 8. Buckman. Plate LXXXII, figs. 5, 6. 
Discoidal, compressed, carinate. Whorls, in section, oblong, ornamented with 
a few irregularly-placed, obscure, direct, upright ribs. Ventral area arched, 
divided by a small, probably solid carina. Inner margin well-marked, broad, 
steeply sloped, tending to become concave with age. Inclusion about five- 
eighths. Umbilicus concentric—the central whorls ornamented with only a few 
small spines and irregularly-placed bullicoste, the outer whorls with plain, 
somewhat distant, rather irregular and obscure ribs. 
Up to about 110 mm. in diameter the form is thin, but afterwards it becomes 
more than proportionately thicker. The same feature is seen in crassinuda a, 
but not to the same extent, because it is thicker to begin with. 
The greater compression of the adolescent shell in relation to the size of the 
umbilicus, and the very little development of spines in the inner whorls, separate 
this form from the varieties of crassinuda ; but the greater proportionate com- 
pression links it to diversa through the form shown in Pl. LXXXI, figs. 5, 6. 
That the immature form is thin and umbilicate seems to negative derivation 
from crassinuda, but to suggest descent from diversa by way of that intermediate 
form. Yet the deficiency of central spimes—the feature which makes levigata an 
aberrant form of the crassa-stock—is a difficulty. Still, im the intermediate form 
the spinous stage is very short, on account of the absence of the irregular spinous 
stage ; and perhaps the law of modification of earlier inheritance would account 
for the development of lxvigata from such a form. This is the best suggestion I 
can make with regard to the genealogy of this peculiar specimen. 
Sonn. levigata is a scarce fossil in the Concavum-zone of Bradford Abbas. <A 
portion of its side view illustrating the coiling and ornamentation of the umbilicus 
is shown in fig. 5 of Pl. LXXXII; and the section of the whorl in outline 
(restored), from a little behind the aperture, is given in fig. 6. 
