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

HOPLITES.
often octagonal shape. It also produces a certain amount of distortion of the
interior space of the shell, producing peculiar variations in the structure of the
suture-line. In any case, distortions of the suture-line, such as occur in Acanthodiscus
octagomts (see plate XXVII, fig. 3c), are caused by the formation of
tubercles. In every form of which the suture-line is known, the number and
usually, the disposition of the suture-line, as well as the number of the principal
inflections, are constant, but the degree of ramification, the length of the branches
and the breadth of their trunks are subject to considerable variations. Ac.
Hookeri and Ac. acanthoptychus possses rather broad lobe-trunks, Ac. subradiatus,
Ac. Sommeringi, Ac. octagonus, and Ac. asiaticus rather narrow ones. The European
Acanthodiscus radiatus 1 and the Indian Acanthodiscus subradiatus are exceedingly
closely allied in every respect; and yet the lobe-trunks of the former areremarkably
broad, those of the latter are very narrow.
Among the Indian forms of Acanthodiscus the group of Acanthodiscus octagonus,
which besides the latter species includes Ac. octagonoides n. sp. and Ac..
poll/acanthus, exhibits the most extreme varieties of shape. The intercalary ribs
disappear here completely or nearly completely, and the sculpture only consists
of strong main-ribs which in the middle stages of growth resolve themselves into
two anteriorly deflected branch-ribs. In the most extreme form, Acanthodiscus
octagonus, one of these two branch-ribs is even suppressed with the consequence
that the body-chamber only bears a few simple main-ribs, each with an umbonal,
a lateral, and a ventral tubercle. The primitive costate stage is traversed
exceedingly rapidly. The tubercles are produced into long massive spines which,
on the chambered portion of the shell, are shut off by a lamella from the interior of the
shell. Tho bodies of the lobes are narrow; in Ac. octagonus some of them are distorted.
In tho largest specimens the body-chamber shows signs of becoming
disconnected from the inner whorls. In connection with this feature it is interesting
to' notice that there exist species of Crioceras closely related by their ornamentation
to the group of Acanthodiscus octagonus, such as Crioceras Roemeri
Neum. et Uhlig, Crioceras varioosum v. Koenen, and Crioceras Stronibecki v. Koenen.
Amongst the forms which are cloBe-coiled according to the manner of normal
ammonites it is only Hoplites Sayni Simionescu from the Berriasiun of the south
of Franco which can be placed here besides the Indian species already referred
to.
The second group includes tho following Indian species:—
Acanthoducui lubradiatuM n. ap.
„ aeanthinut n. ap.
,, hundesiantu n.
„ Sdmmiringi Oppel ap.
„ n. sp. ind. aff. tubradialus n. ap.
(T) Medea (Strachey) Blauf. ap.
l 6m Nenouyr ud Utallg, HiUammoniliden, PL 34, fig. 26.
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