SONNINIA SPINOSA. 365 
Sonninia spinosa, S. Buckman. Plate LXXXI, figs. 7—9. 
Discoidal, compressed, hollow-carinate. Whorls ornamented with rather 
small, direct, nearly upright, ventrally-inclined ribs of different sizes: of the 
larger, which occur at irregular intervals, each one carries a median spine, and 
often bifurcates beyond it. (The regular spinous stage persists until the specimen 
is about 25 mm. in diameter.) Ventral area arched, carrying a small, fairly-defined 
hollow carina. Inner margin fairly defined, steeply sloped. Inclusion about 
three-eighths—almost up to the spines. Umbilicus concentric, somewhat deep, 
graduated, ornamented with ribs and spines. Whorl-section elliptical, about one- 
fifth indented. “Suture-line with a symmetrical superior lateral lobe; the lateral 
lobules equipoised, the terminal lobule axial and isosceloid. 
Ata fairly early period of growth the spines are long, rather taper, and sharply 
elevated ; but later on they decline in elevation, so that the ribs which are only 
knobbed, would be technically called bullicostz.' The spines or knobs are set 
rather on the inner half of the whorl, so that, though the inclusion is considerable, 
there is still a slight space between the spines and the inner margin of the 
succeeding whorl. ‘The position of the spines and the amount of inclusion give a 
small umbilicus, relatively to similarly ornate species, in correlation with somewhat 
elaborate ornament ; and such a combination of characters at once distinguishes 
this from any of the correspondingly spinous species described. 
Sonn. spinosa is the morphological equivalent of acanthodes, irregularis, and 
crassiformis ; but it is separable from any of these by the combination of characters 
mentioned above, as well as by the smaller size of its spines. Further, it is 
distinguished from acanthodes by its upright ribs and greater inclusion; from 
wregularis by less elaborate ornament ; from crassiformis by the spines not being 
regular—ribs with and without spines being somewhat irregularly alternate in the 
present species. 
The specimen figured as the type is not adult. In my collection there is an 
example 195 mm. in diameter, and it is septate to 176 mm.; and therefore it 
would probably have measured 240 mm. when complete. ‘There is no reason to 
suppose that this is the largest size to which the species attained, though it agrees 
fairly with the size of allied species. This specimen shows that there is further 
decline in the size of the bullicoste, and then a slight increase later on. It 
suggests, too, that the last knobbed rib shown in the type (just under fig. 9 of the 
plate) is possibly abnormal. Further, it affords evidence that without the test the 
bullicostee are not to be observed—the specimen appears to have merely plain 
1 On the ribs which bifureate the elevations are more like nodi. 
