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

HOI'UTKS. 1*1
7 mm. find again Wonting gradually otwolete, after the diameter ha.
23 mm.
Amongst tho species which are immediately allied to Blanf. WaUvM tU
ventral furrow remains distinct as far as the neighbourhood of the body^haniber,
and only at that Btage does the furrow flatten out and become indiutinrrt, allowing
tho ribs to traverse the external margin uninterruptedly, although more or
less reduced. In other forms the furrow becomes obliterated at a somewhat
earlier age, but even in the largest-size specimens it is indicated bv the rweling
of the ribs on the external margin. The external tuberclee are distinctly developed
in only a few forms, such as Bhnf. acuticoata, nevertheless their presence
is suggested by the occurrence of slight rib-swellings which can be observed even
in large specimens.
Umbilical tubercles are not developed in any of the forms belonging to the
present group, nor is it possible to discover, even in a single case, a distinct
union of two main-ribs at the umbilicus.
In all forms of the genus Blanfordia the relation of thickness to height of
volution changes in the course of ontogenetic development in this sense that with
increasing size the thickness increases comparatively more rapidly than the
height. This statement is not only applicable to the species from the Spiti
Shal es, but according to G. Boehm holds good also with respect to the fonns
from the Dutch East Indies. It is true that the relatively more rapid increase
in thickness is, as a rule, not very considerable, not, for instance, as consider-
able as in the genus Himalayites; it is indeed only slight in some species,--but
nevertheless noticeable. The cross-section of the volutions of many species is
elliptic, at least during the earlier stage, but starting from this as the primitive
form the cross-section may become now trapezoidal, now subquadiatie, now
roundish-clepressed in outline. The most strongly inflated species, such as Blanf.
Celebrant and Blanf. Middlemissi, possess fairly thick volutions even when quite
young. The umbilical wall is that part of the shell which is least aflected by
those changes ; in all the various forms it is slightly rounded, and it is never
distinctly, marked off from the flanks, whilst its slope towards the wide, often
funnel-shaped umbilicus is almost always more or less oblique, seldom steep.
The suture-line exhibits none but unimportant variations. The external
and the first lateral lobe are of about equal length. The first lateral lobe has a
massive trunk which ramifies sub-symmetricallv giving origin to a slender terminal
branch and to two principal as well as to several secondary lateral branches.
The second lateral lobe is much shorter than the firat one ; in the more slender
forms of the genus its disposition is somewhat more oblique and its conforma-
tion is somewhat more unsymmetrical than in the strongly inflated species. In
the thick forms—Blanf. Celebrant and Blanf. sp. ind. G. Boehm—the second
lateral lobe possesses a short and broad trunk and a remarkably long and narrow
terminal branch. In almost all forms the external saddle is divided into two
j»rts by a long secondary lobe. These two parts of the external saddle are of