HAPLOPLEUROCERAS SUBSPINATUM. 
301 
With the species of the Liassic genus " Pleuroceras " the present species is 
most likely to be confounded ; but it is more compressed and more umbilicate 
than any of them. It is most like " hawskerense" but that is a thicker shell, and, 
moreover, has tubercles or very swollen ribs on the ventral area between the 
outer lateral row of spines and the ventral sulci, so that it becomes a fossil of the 
" multispinous stage." 
It will be noticed that there are at least three varieties of this species figured 
in the plate : a strongly spinous form (fig. 1), which has the spines of its inner 
whorls fitting into niches in the succeeding whorls ; a finer-ribbed form, with 
spines less marked — this is partly due, perhaps, to inferior preservation (fig. 6) ; 
a finer-ribbed form with broader whorls and obsolete ventral furrows (figs. 8, 9) ; 
while the small specimen in fig. 4 shows more distant ribs than the umbilicus of 
fig. 1, and probably indicates a still more widely-costate variety. 
Some features of this species demand attention ; for instance, the occasional 
small, non-spinous intermediate ribs — the tendency of the ventral strias to become 
very pronounced, so as to give the appearance of a breaking up of the ribs in the 
ventral area (fig. 10) — the slight crenulation of the carina (fig. 10). The last is a 
most extraordinary instance, of the development of homoplastic features, if the 
descent of this genus be as I have now suggested. 
Haplopleuroceras subspinatum is not a rare fossil, but it seems to be very local. 
It is practically confined to a small area — a two-mile radius from Bradford 
Abbas, being a characteristic species of the Concavum-zone of that district. 
Dr. Haug tells me that he has found this species in the neighbourhood of Digne 
associated with the following Ammonites : 
Phylloceras Nilssoni, Heb. Lioceras concavum. 
,, ultramontanum, Zittel. Ludwigia cornu. 
,, trifoUatum, Neum. Hyperl. Walkeri. 
Lytoceras pygmseum, d'Orb. Steplian. pundum, Vacek. 
Sonninia cf. acanthodes. Erycites fallax, Benecke. 
Figs. 1, 2, PI. LI, give two views of a strongly-spinous form ; fig. 3 is part 
of the suture-line. Figs. 4, 5, are two views of a coarse-ribbed young specimen. 
Fig. 6 is a side view of an example with less developed spines ; fig. 7 is its suture- 
line, with a peculiar twist, to the superior lateral lobe, which presumably is not 
normal. Figs. 8, 9 furnish two views of a finer-ribbed variety. Fig. 10 is an 
enlarged view of the ventral area. Figs. 13 — 15 give views of a pathological 
example of Haplopleuroceras, probably of this species. The injury has caused a 
sudden retrogression to a costate stage with planicostan abdomen, presumably a 
very decided case of atavism. Figs. 5 and 6, PI. XLIX, represent the young stage. 
