108 
NAUTILOIDEA. 
“ The typical forms of this genus are distinguished by the widely 
umbilicated, slightly embracing whorls, and by the peripheral and 
lateral longitudinal sculpture. This sculpture consists either of 
simple longitudinal ribs, or they may be nodose or crenulated. In 
a number of Carboniferous forms the ornaments become obsolete in 
the adult age, approaching thereby the closely -allied genus PJeuro- 
nautilus. 
“ The Triassic forms agree in part (Trematodiscus c/emmatus) 
with the Carboniferous primitive form, and in part they deviate 
from this type in the disappearance of the lateral sculpture. 
“ The majority of Carboniferous forms have simple, straight septa. 
In the Triassic forms a flat external, and a flat but broader lateral 
lobe is present. In the majority of the Carboniferous forms an 
internal lobe is wanting. It is found, however, in Xautilus cari- 
niferus, de Koninck {non Sowerby), and in Gyroceras hinodosum, 
according to Sandbergerh 
“ In the Triassic forms the occurrence of an internal lobe has 
been shown in T. gemmatus^ and T. rectangidaris'\ 
“ There can hardly be any doubt about the origin of the genus 
Trematodiscus if one compares the figures of Gyroceras consohrinum, 
G. intermedium, G. propinquum, G. tessellatum, and G. serratum in 
de Koninck’s fine Monograph ‘ with those of the Trematodisci con- 
tained in the same work on pis. xxvii.-xxx.^ These two series of 
forms are only distinguished by the different amount of the enve- 
loping of their whorls. As de Koninck himself has justly observed, 
it is impossible to distinguish the first whorl of Trematodiscus from 
that of a Gyroceras belonging to the series above enumerated. To 
these also manifestly belong the slightly curved forms that de 
Koninck has described [Calc. Carb. pL.xxxiii.] under the names 
Cyrtoceras canalicidatum, G. Puzosianum, and 0. Gesneri.” (Mojsi- 
souics.) 
The species belonging to the X3resent genus recorded by Mojsisovics® 
are the following: — Trematodiscus \_Coelonautilus'\ rectangular is, F. 
von Hauer‘S, T, [(7.] Klip>steini, Mojs., and T. [C.] gemmatus, Mojs. 
In examining the remarkably fine series of examples of Coelo- 
nautilus cariniferus, mostly from the Carboniferous Limestone of 
’ It is present also in Discites discors, M‘Coy. 
^ Das Gebirge um Hallstatt, 1873, Theil i. pi. iii., 
^ Die Cephalopoden der Mediterranen Triasprovinz, 1882, pi. IxxxTii. 
^ Faune dii Calc. Carb. de la Belgique, 1880, pi. xxxiii. 
3 Ibid. 1878. 
® Die Cephalopoden der Mediterranen Triasprovinz, 1882, p. 270. 
’’’ Denkschr. d. k.-k. Akad. d. Wiss. zu Wien, Band ix. p. 145, Taf. i. ff. 1-4. 
