SONNINIA DENSICOSTATA. 375 
view reduced one-half; fig. 8, outline of the front view similarly reduced; fig. 9, 
portion of the suture-line. 
Sowninia DENSICostTATA, S. Buckman. Plate LXXXVIII, figs. 8, 9, 9a. 
Discoidal, compressed, hollow-carinate. Whorls ornamented with rather 
small, well-marked, direct, slightly reclining, ventrally-inclined coste. (Here and 
there a rib is bigger than its fellows, and at from 30 mm. up to about 40 mm. 
diameter some of these ribs carry small knobs. At about 120 mm. diameter the 
knobs appear again, and the rest of the fossil is ornamented with slightly bullate 
costz. A few ribs are connate at the inner margin.) Ventral area subacutely 
arched, medianly flattened, divided by a well-defined, somewhat large, laterally- 
compressed hollow carina. Inner margin broad, fairly defined, subconvex, the 
upper edge much rounded off, especially in inner whorls. Inclusion about one- 
fourth. Umbilicus concentric, shallow, open, costate, graduate, with gibbous 
whorls. Whorl-section elliptical. 
This species is very different from any other that has been figured in this work, 
with the sole exception of Sunn. ewromphalica (Pl. LXXXV, figs. 1—3). Com- 
pared with that species, it lacks the central spinous stage, has ribs of more 
unequal size,and, though comparison is difficult, the outward shelving of the 
umbilical whorls indicates a more compressed ventral area. 
The absence of spines in the inner whorls is a feature of importance. The 
details of the centre are wanting, but at a radius of about 5 mm. there are plain, 
slightly-reclining costz, rather closely set. At a radius of about 13 mm. the 
costz become somewhat irregular, and are occasionally connate in pairs on the 
inner area, the connate pairs parted by one or more single ribs—an ornamentation 
like that of biplicata (see page 345). About half-a-whorl later some of the paired 
ribs tend to become slightly bullate at their middles. For about another half- 
whorl every third rib is shghtly larger than its fellows, and very slightly bullate— 
the connate character being obscure, though the ribs are somewhat paired. At 
about 70 mm radius some ribs become somewhat medianly bullate; and at about 
80 mm. radius nearly every rib is furnished with a small elongate knob: in the 
last ribs this character tends somewhat to decline. 
The above details, and others mentioned in the specific diagnosis, separate 
this species from any of the costate forms, Sonn. costata, &c., which agree with 
it in the absence of spines. There are two foreign species to which it bears 
more resemblance—Witchellia Sayni, Haug’ [? Sonninia] and Sonn. polyacanthus 
1 Ludwigia corrugata, Douville (non Sowerby) “ Zone A. Sowerbyi,” ‘ Bull. Soc. géol. France,’ 
