SONNINIA BIPLICATA. 345 
Ventral inclination of ribs (or growth-lines) more marked : 
S. biplicata B. More spinous, less costate, and more involute than biplicata a. 
S. alternata B. Very similar to alternata a, but the ventral inclination of the 
ribs (or growth-lines) much more marked.’ 
The genetic affinities of the biplicata-stock are very easily determined. The 
very strongly-reclining ribbing brings it into relationship with the acanthodes- 
stock ; and the gibbous-sided whorl is a confirmatory feature. The irregular and 
regular spinous stages point in much the same direction as the same features of 
acanthodes, &c., to a spinous form like crassispinata. The differences in the 
costate stages prevent derivation from acanthodes; but the similarity of the 
Spinous stages points to a common origin for biplicata and acanthodes. 
From external characters it would appear that biplicata a developed into 
alternata a, and biplicata B into alternata 3; but it has been found that the 
duplication of the outer lobule of the superior lateral lobe in biplicata a is much 
too pronounced to allow this view to be held. Possibly, before the descriptions 
of Sonninie are finished, the advent of more material will enable me to treat 
more critically the forms now grouped in * the biplicata-stock.”’ 
Sonninia Breticata, S. Buckman. Plate LX XVIII. 
Discoidal, compressed, hollow-carinate. Whorls, in section, circular to elliptical, 
ornamented, after the usual regular and irregular spinous stages, with strongly- 
marked, reclining ribs, arranged first in pairs—the more-marked and the less- 
marked,—later, an obscure and well-marked rib in alternation: the pairs of strongly- 
marked ribs approximate, and with a tendency to be connate on the inner area.” 
Ventral area obtusely arched, slightly flattened in the middle, which is divided by 
a fairly-marked hollow carina; the ribs on the sides of the ventral area rather 
obscure, ventrally-inclined. Inner margin nearly upright, subconvex ; but it is 
scarcely defined, as the upper edge is so much rounded off. Inclusion about one- 
third. Suture-line rather ornate, the superior lateral lobe tetradactyloid on account 
of bipartition of the outer lobule. 
1 Viewed from behind, the ribs (or growth-lines) of alternata a join the carina at an angle of 
about 50°, those of 8 at 25°. 
* The pairs of strongly-marked ribs are doubtless homologous to spines—in fact, they may be 
regarded as aborted spines. Often, in the irregular spinous stage of the Sonninie, a single rib leads 
up to the spine and bi- or trifureates beyond. In the present case it may be said that the spine is 
transformed into a kind of duplicated primary rib. 
AE 
