346 INFERIOR OOLITE AMMONITES. 
This description applies to the type only; but I have separated two forms 
under the name biplicata, thus : 
a. The type (Pl. LXXVIITI, figs. 1—3). 
B. The spinous stage rather longer, the succeeding coste not so marked, the 
ribs rather more turned on the ventral area (Pl. LXXVIII, figs. 4, 5). 
The peculiar arrangement of the ribbing in the costate stage effectually 
separates this species from any other I have described. At first the ribs are of 
two sizes, the larger being very strongly marked. The more-marked ribs are 
arranged somewhat in pairs, roughly alternating with pairs of the less-marked ribs ; 
and, so far as regards the strongly-marked ribs, the space which connects the ribs 
forming the pair is often elevated, so that the pair at the inner end look somewhat 
like a broad single rib medianly-sulcate. Later on, the ribs are single, and an 
obscure rib alternates with one well-marked. 
The ornamentation above described is not so noticeable in biplicata B, because 
it has barely finished with the spinous stage ; but somewhat the same “ alternation” 
may bedetected. The Concavum-zone of Bradford Abbas has yielded the examples 
of this rare form which are in my cabinet. The form a is illustrated in PI. 
LXXVIII, figs. 1—3, and the form f in figs. 4, 5 of the same plate. 
SonNINIA ALTERNATA, S. Buckman. Plate LXXVI, figs. 7—9; Plate LXXVII, 
figs. 3—5. 
1881. Harprocrras apicrum, S. Buckman (non Waagen). Inf. Oolite Ammo- 
nites; Quart. Journ. Geol. Soe., vol. xxxvii, 
p. 603 (pars). 
Discoidal, compressed, hollow-carinate. Whorls, in section, elliptical, orna- 
mented in the costate stage with irregular-sized, reclining ribs, at first somewhat 
in pairs, later on alternate.!_ Ventral area subobtusely arched, divided by a fairly- 
prominent hollow carina. Inner margin well-defined, nearly upright. Inclusion 
rather less than one-half. 
As alternata I have separated two forms; but, practically, the only difference 
between them is that in the form B (Pl. LXXVI, figs. 7—9) the ribs (or growth- 
lines) are more ventrally-inclined—they might almost be called ventrally-projected 
—than in the form a (Pl. LXXVII, figs. 3—5), in which they are only inclined. 
In Sonn. alternata the spinous stage ends rather earlier than in biphicata. In the 
1 See description of preceding species. 
