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

HIMALAYITES. 137
On the inner volutions the ribs have a slightly sigmoidal form, and the branch-
ing-point of the ribs lies at a somewhat higher level than on the outer volution*.
On the innermost whorls the majority of the ribs are bifurcate, the number of
simple ribs being very small. On the outer volution the ribs take a radial
course, and they traverse the ventral face in the shape of exceedingly sharp
ridges.
The main-ribs swell up into strong tubercles, and each gives rise to three
to five branch-ribs. Between the main ribs are inserted two to three simple,
more rarely branching intercalary ribs. Along the median line of the external
margin the ribs are slightly grooved.
The suture-line has a fairly simple structure. The first lateral lobe is only
slightly shorter than the external lobe, the second lateral lobe is disposed Blightly
obliquely. The two main saddles are cut into by a rather short secondary lobe,
their bodies are broad and only slightly encroached upon by secondary lobes
and the lateral branches of the primary lobes.
The full-grown stage and body-chamber are not known.
H. Sioliczkai is distinguished from H. kutianus particularly by a much earlier
appearance of the tubercle sculpture and by the greater number of branch-ribs
which go to form a bundle ; from H. Seideli it differs by the same character-
istics as well as by its greater thickness and less ramified lobes ; from H. ventri-
cosus it is distinguished by its slower rate of increase in thickness, the develop-
ment of much stronger tubercles on the inner volutions, the smaller number of
intercalary ribs and the greater number of branch-ribs and finally by the
different structure of its suture-line.
Amongst European species H. Cortazari Kilian sp. resembles H. Sioliczkai
in such a degree that the two might be united, if we adopted a wider concep-
tion of species. But differences do exist. H. Cortazari bas somewhat thicker
volutions ; the number of branch-ribs forming a bundle is somewhat smaller
and the ribs are somewhat less curved than in H. Sioliczkai. As long as no
marked transitions between the two species have been discovered, it will be
advisable to keep them separate.
The relationship of H. umbo Strachey-Blanford to the present species cannot
be stated with certainty, because of H. umbo only a small fragment of one of
the larger volutions is known.
H. Stoliczkai was found near Gieumal.
HIMALAYITES UMBO, Strachey et Blanford sp.
1866. Ammonites umbo, Paleontology of Niti, p. 78, plate 17, fig. 2.
The fragment which is 50 mm. lorg and which Blanford described under
the name of Ammonites umbo from the^Spiti Shales of Niti, although representing
an independent species, is not such as to permit of a satisfactory specific
diagnosis. H. umbo differs from H. Seideli by its greater thickness, from