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

HIMALAYITES. 137
there, we are certainly in a position to declare that the predominant increase ir,
the thickness of the volutions characterises the main phase of ontogenic development.
Simultaneously with the changes in the cross-scction of the volutions there
occur changes in the ornamentation. The innermost volutions do not yet posse*
the sculpture described above, but are ornamented with fine and sharp rite
which run for some distance across the flank; either in a radial direction with a
slight anterior deflation, or else in a somewhat sigmoidal curve, and which, after
having formed a swelling, split up into two branch-ribs which terminate each in
ft fine, pointed, small tubercle on either side of a sharp median ventral groove.
At this stage the sculpture recalls that of Parkinsonia as well as the immature
forms of the group of Hoplites Callisto and Hoplites Wallichi. As the shell grows
in size, the ribs- become gradually differentiated into tuberculiferous ribs and simple
non-tuberculate ridges. The point at which the ribs divide and which in the
immature stage is situated near the external margin of the volution shifts its
position to about the middle of the flanks, and the knots at the point of bifurcation
increase very rapidly in size. In the full-grown stage the tubercle-bearing ribs are
somewhat stronger than the non-tuberculate ones. It is only when two ribs unite
to form a tubercle that this strengthening of the ribs does not take place. The
median furrow of the immature stage is soon lost; when the diameter has
reached 25 to 35 mm., the ribs, where they cross the median line of the ventral
face, are intersected by a very shallow groove or only slightly reduced, and
finally only a trace of this reduction remains visible.
This course of development is followed by different species to a different extent and
is passed through more or less rapidly or slowly. Him. hyphasis Blanford (non
Stoliczka) represents a primitive type : it is a small species with relatively high
volutions in which development proceeds up to the differentiation into tuberculate
and non-tuberculate ribs, but does not reach the stage where trichotomy of the ribs
sets in. Only in one place does a specimen show trifurcation of a costa, and
tubercle formation is yet little pronounced. Whilst, therefore, Him. kyphosis
proceeds only slightly beyond the Parkinsonia stage and retains its primitive
features nearly throughout life, the opposite is the case with Him. Stoliczkai: here
the Parkinsonia stage is rapidly traversed; already at an early age tubercles
make their appcarancc at the branching-points of the ribs, and trichotomy sets
in as soon as the diameter has reached 15 mm. With a diameter of 42 mm. the
same species exhibits a division of a rib into five branch-ribs. When of a diameter
of 35 mm. Him. ventricosus is yet in the Parkinsonia stage, but suddenly and
practically without passing through a transitional stage it develops strong tubercles
and the breadth of its volutions increases at a rapid rate. In Him. Scldagintweiti,
also, the Parkinsonia stage is only abandoned when the diameter has reached 43
mm., but with a diameter of 29 mm. isolated trichotomous ribs already make
their appearance. Him. kutianus is yet in the Parkinsonia stage when measuring
22 to 30 mm. in diameter, but at 35 to 40 mm. it develops a soulpture which
corresponds approximately to the developmental phase of Him. hyphatis ; it is only
B 2