350 INFERIOR OOLITE AMMONITES. 
adult ; and ought, perhaps, to be regarded as the morphological representative of 
a still earlier adult ancestor connecting multispinata with crassiformis. 
I have no other example in my cabinet. The Concavum-zone of Bradford 
Abbas is its horizon. 
Sonnrnza ORASsA, S. Buckman. Plate LXXXII, figs. 1, 2. 
Discoidal, compressed, carinate. Whorls, in section, oblong, ornamented 
with rather close, not very prominent, direct, upright, undulate ribs,‘ inclined, 
but obscure ventrally (the spinous stage only in the inner whorls). Ventral 
area rounded, divided by a small, presumably hollow carina. Inner margin broad, 
well-marked, subconvex, nearly upright. Inclusion about one-third, apparently 
more in youth. Umbilicus rather deep, somewhat excentric. 
The external characters suggest that this species is the descendant of crassi- 
formis—separated by some distance. It is decidedly much further removed from it 
than is crassibullata (page 353), the less elaborate spinous stage shows that ; yet it 
has in general appearance a greater likeness to what would be expected as the 
development of crassiformis. The connecting link I assume to have been stronger 
and more closely costate than crassibullata ; for crassa shares with crassiformis 
greater regularity of ornamentation and a more developed costate stage than are 
found in crassibullata. Certainly the coste of crassiformis are really bullate, but 
an indication of the same feature is seen continued in the coste of this species :° 
they are certainly elevated in the middle. 
So far as the costate stage is concerned, Sonn. crassa is the morphological 
equivalent of Sonn. dominans,’ to which it bears considerable resemblance. Not 
only, however, is it a considerably thicker shell, but in youth the spines are 
more numerous, and they last to a much later date; in fact, so far as the spinous 
stage is concerned it is not nearly so retrogressive as dominans. 
This species occurs in the Concavwm-zone. It does not appear to be common, 
but perhaps it has been passed over on account of its resemblance to dominans. 
1 The ribs increase in size considerably on the last quarter-whorl, and are altogether more 
prominent. The core of the body-chamber shows some obscure, inclined, rib-like markings on the 
ventral area between the true ribs. 
2 There are distinct bullicoste in crassa between the true spines and the true ribs, but they are 
rather obscured by the involution. 
3 A small difference should be noticed. The ribs of dominans are more nearly of one size across 
the whorl; the ribs of crassa are raised in the middle, but rather obscure both on the inner and outer 
lateral areas. 
