SONNINIA CRASSIFORMIS. 349 
not quite so far. Termination, adult, a plain edge, parallel to ribs and growth- 
lines. Suture-line with a tetradactyloid superior lateral lobe, the outer lobule 
being in duplicate. 
This species is one of the giants of the Concavum-zone, and is the largest of 
the spinous Sonninizx. The splendid adult example (figs. 3, 4), which is complete, 
measures 290 mm. (113 inches) in diameter. 
Biologically this species is the morphological equivalent of acanthodes and 
iregularis. From the former it may be distinguished by the whorls overlapping 
to the spines for a longer time, by greater regularity of ornament, and by the more 
upright ribs; and from the latter by greater regularity of ornament—the regu- 
larity of the spines being particularly noticeable,—by its large bullicoste, 
and by its more inflated whorls. It will thus be evident that from the characters 
of their side-surface alone these three species are distinguishable thus : 
S. acanthodes, ornamentation somewhat irregular, ribs very strongly 
reclined. 
S. irregularis, ornamentation very irregular, ribs nearly upright. 
S. crassiformis, ornamentation very regular, ribs nearly upright. 
In regard to descent, there is no evidence, in the inner whorls, of that irregular 
spinous stage which is seen in crassispinata; but this possible stage in the 
phylogeny may have been omitted from the ontogeny by the tendency to dispense 
with dissimilar stages. On the other hand, it is as reasonable to suppose direct 
derivation from multispinata—an enlargement of spines with more inflated whorls ; 
and this agrees with the ontogeny. 
Sonn. crassiformis 1s a scarce species in the Concavum-zone of Bradford Abbas. 
An immature example of the species showing the regularity of the spines is 
depicted in figs. 1, 2, Pl. LX XIX; and in figs. 3, 4 of the same plate are copies of 
two photographs of the side-views of a giant adult, reduced, being 95 mm. against 
290 mm. in diameter. The outline of the aperture, and the suture-line of this 
fossil are shown in figs. 5, 6, both of natural size. 
Sonninia, sp. Plate LXXIX, figs. 7, 8. 
In the same plate with the figures of Sonn. crassiformis I have placed a small 
fossil on account of its resemblance to that species in whorl-shape; but it has 
Spines too small and too close together to be the young thereof. In the arrange- 
ment of its spines it is rather like multispinata, but has a more arched ventral 
area and differently-shaped whorls. Itis probably a young example of an unknown 
