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

HOPLITES. Jtf!
intermediate ribs are not yet completely obliterated, being partly «tdi *»
feeble traces. Consequently here also it is not po*»ible to draw a ryfirt
ary line. Of European forms Acanthodiscus perwnatus Retownki, Ac. Ruttmefer*
Sarasin et Schondelmeyer (non Oster), Ac. sub-Chaperi Sar. et Hckamldiu..
Ac. hystricoides Uhlig appear to belong to the present group.
Still less sharply marked off is the fourth group including the, {following
Indian species:—
Acanthodiscus spitiensis n. sp.
» aff. Michaelis^JhMg sp.
„ tibetanus u. sp.
i, asiaticus n. sp.
» aff. asiaticus n. sp.
» n. sp. ind. aff. spitiensis n. ap.
„ Ruprechti Oppel sp.
In these forms the rib-sculpture still further predominates over the tuber-
culatum, which sinks to the position of a subordinate character. The number
of intermediate ribs amounts to three or four. The volutions are higher and
more slender than in the other groups, the lobe trunks more copiously ramified.
In several species the general external appearance is determined bv the ribs
and not by the spines, and the latter appear to differ considerablv from those of
the preceding groups. But here again we meet with intermediate forms which
establish a close connection with the other groups. Such species as Acanthodis-
cus aff. hystricoides, which has been enumerated in the preceding group, might
just as well have been placed in the fourth group. In Europe the present group
is represented by Acanthodiscus Michaelis Uhlig sp., Ac. Hohennegeri Uhlig sp..
Ac. Paquieri Simionescu sp., Ac. svb-Chaperi Retowski (non Sarasin et Schon-
delm.), Ac. pseudo-Malbosi Saras, et Schondelm., Ac. incompositus Ret. sp., Ac.
incompositus discrepans Ret. sp., Ac. Riitimeyeri Oost. p.p.
Notwithstanding considerable differences in details, the four groups which
we have just discussed constitute a well circumscribed systematic unit. A single
form, however, Acanthodiscus himalayanus n. sp., occupies a position apart from
the others and cannot be united with any of the other groups. Its anterior
extremity exhibits a trituberculate ornamentation, but the inner portion of the
shell up to a diameter of 56 mm. is beset with bundles of unbranched ribs
originating in the umbilical tubercles and exhibiting the tvpical Aulacostephanus
sculpture. Only two of the species of Hoplites from the Neocomian of Europe,
namely, Hoplites hystrix (Bean) Neum. et Uhlig and Hoplites spiniger v. Koenen
from the Hils formation of Northern Germany, exhibit similar features.
With regard to these remarkable forms, we have to choose between twv»
alternatives: either the Aulacostephanus sculpture of the inner whorls indicates
the phylogenv, or the trituberculate ornamentation of the middle and final stages
must be taken as a guide. On the former supposition, the trituberrulare
rj