Ser. XV. HIMALAYAN FOSSILS, Vol. IV. THE FAUNA OF THE SPITI ...

HIMALAYITES. 137
the imperfection of the fragment prevents from being sufficiently well characterised.
The fragment in question is, however, interesting enough to deserve a
brief reference.
It corresponds to that early stage of development in which there still subsist« a
shallow ventral furrow and the tuberculiferous ribs divide only into two branch-ribs.
The number of intercalary non-tuberculate ribs fluctuates between one and three.
Remarkable is the fact that every one of the posterior branch-ribs undergoes a
slight thickening at the ventral furrow, which creates a certain resemblance with
Hoplites, a resemblance which is further accentuated by the rounded-oblong crosssection
and the flattened flanks. The generic position cannot be ascertained
with complete certainty owing to the small size of the fragment, and its place
among the species of Himalayites has been assigned only provisionally.
The thickness of the volution is 17 mm. at its anterior end, its height Ls
9"5 mm.
The suture-line is constructed according to the Himalayites type. The
intermediate and final stages of growth are unknown. Supposing that the specimen
belongs to a species of comparatively small size, then H. kyphosis must be
considered its closest ally. The differences are enumerated in the description
of H. kyphosis. The present species is distinguished from Ammonites microcanthus
Oppel by its greater breadth of volution and by the swellings of individual
ribs on the ventral face.
The specimen was found in the Spiti Shales.
HIMALAYITES HOPLITIFORMIS, n. sp.
(Plate XLII, fig. 2 a—c.)
This species deserves our special attention, because it is the only one which
represents a mutational tendency, the direction of which deviates considerably
from that indicated by any of the other forms constituting the genus Himalayites.
This is the reason which has prompted us to bestow a name on the present
species, although the available material is very scanty and permits only
a partial diagnosis. The specimen consists only of a fragment of a body-chamber
at the posterior end of which one can recognise the presence of a slender second
lateral lobe, placed at a comparatively high level, and traces of a broad lateral
saddle and a retrograde umbilical lobe. The cross-section has a rounded-trapezoidal
form; the flanks are flattened towards the external margin and descend
at rather a steep slope towards the umbilicus. The maximum thickness lies below
the middle of the volution. The ribs are strongly developed and slightly
falciform ; they traverse the external margin either normally or in a gentle arch,
and are slightly weakened along the median line. Individual ribs swell up into
a tubercle just below the middle of the volution and two branch-ribs originate
D 2