DUMORTIERIA GRAMMOCEROIDES. 263 
Discoidal, compressed, carinate. Whorls ornamented witli more or less 
reclining, direct to subarcuate, ventrally-inclined, sometimes bifurcate ribs. Ventral 
area fairly defined, flattened, scored laterally by the ribs, divided by a small, 
inconspicuous carina which is bordered by a narrow, smooth area each side. (In 
aged examples the ventral area is much flattened, the carina practically obsolete, 
and obscure ribs cross the area with a slight forward inclination.) Inner margin 
well marked in adult, smooth, straight. Inclusion about one-third. Umbilicus 
open, scored with numerous ribs. Termination, in youth two lateral and spatulate 
ears, and a very short, blunt ventral process ; in adult probably a plain sigmoid 
bend. Suture-line somewhat ornate ; a well-marked superior lateral lobe longer 
than siphonal ; inner lobes much dependent. 
From an extensive series of this species I have figured an interesting selection. 
The chief forms may be referred to as the less-developed — the evolute, wide-ribbed 
form (PI. XL VI, figs. 6, 7) ; and the more developed — the involute, close-ribbed 
form (PI. XLVI, figs. 4, 5) ; but between these two are all the possible forms 
which all different combinations of these characters could produce, for hardly 
two specimens are quite alike. 
Adult examples of this species, like the fine one shown, PI. XLVI, figs. 1, 2, 
are not at all commonly found — a contrast to the specimens about three inches in 
diameter, which are fairly numerous. The adults have several peculiarities not so 
noticeable in smaller examples, namely, the well-defined inner margin, the obscure 
bifurcation of the ribs, the absence of the carina in the last whorl, and obscure 
ribs then crossing the ventral area in a somewhat flattened-out, irregular fashion, 
and with a slight forward projection. 
A noticeably peculiar variety of this species (PI. XLVII, figs. 13, 14) has 
strongly reclining ribs and a carina more than usually prominent. As ribs and 
carina are very variable in the species, I will remark on their chief characters 
generally. 
The Bibs. — Roughly speaking, the evolute forms have the direct ribs — the true 
Dumortierian ribs — sometimes inclined. The more involute forms have slightly 
reclining ribs, with more or less of a lateral bend, so that the ribs are sub- 
sigmoid. The specimen figured in PI. XLVII, fig. 13, shows the character 
of the reclining ribs very much pronounced. It is to be noticed that, at intervals, 
it changes the slope of the ribs to a more direct position, by the insertion of a rib 
or ribs on the outer area ; but after the change the ribs gradually fall back more 
and more until the next change, when they are, so to speak, suddenly righted 
again.^ The strongly-marked inclination of the ribs does not give rise to any 
' Whatever process o£ growth and absorption produced this mode of ornamentation it was 
evidently the same as produced a similar state of things m Morpkoceras, although in that case 
accompanied by periodic constrictions. 
