354 INFERIOR OOLITE AMMONITES. 
parallel developments from an ancestor thus morphologically represented ; and 
possibly the young specimen shown in Pl. LXXIX, figs. 7, 8, may, by morpho- 
logically representing the adolescent stage of that ancestor, and the adult stage of 
one still earlier, furnish a lower link in the series. 
Next to Sonn. obtusiformis the present species is one of the most distinctly 
marked of any of the Concavum-zone Sonniniz. Its upright ribs separate it from 
any morphological equivalents except marginata; from which it may be known not 
only because its coste retrograde more quickly, so that there is a greater contrast 
between the strength of ornamentation of inner and outer whorls, but because 
the strength and arrangement of the ornamentation is unlike at all stages of 
growth. 
Sonn. crassibullata is 2 somewhat rare fossil in the Concavum-zone; but it is 
always likely to attract attention because it can be so readily identified. All the 
specimens in my possession came from Bradford Abbas. The side view of a 
grand adult shell is shown in Pl. LXXX, fig. 1, the section of its whorl in fig. 2, 
and the suture-line from another specimen in fig. 3. 
Sonninsa orasstoostaTa, S. Buckman. Plate LXXX, figs. 4, 5; and suture-line of 
an intermediate form, fig. 6. 
Discoidal, compressed, carinate. Whorls, in section, oblong, ornamented with 
spines passing to bullicostz, which yield at an early age to somewhat distant, 
ill-marked, direct, upright coste. Ventral area arched, divided by a small, pre- 
sumably hollow carina. Inner margin well-marked, fairly upright, flat. Inclusion 
about one-half. 
The description of this species is somewhat like that of crassibullata—the 
difference lies in the age when the changes of ornament were assumed. In 
crassibullata subbullicoste are present at 190 mm. diameter; in crassicostata 
these subbullicostz yield to plain costz at about 70 mm. diameter; and at about 
100 mm. in diameter the costz are becoming quite obsolete. The young crassi- 
costata is the morphological representation of adult crassibullata. 
This species is connected with crassibullata by a series of intermediate forms. 
It is thinner than that species, and is easily distinguished by the reduction of 
ornament. It cannot be confounded with any other Sonninia; it is far more 
spinous than domimans, or crassinuda; it is thinner, more regularly spinous, and 
then smoother than crassa. 
It occurs in the Concavum-zone of Bradford Abbas, but is not common. 
