NAUTTLIl)^. 
135 
Remarl'S. “ I was at first disposed,” says Mojsisovics, “ to make 
PleuronantUus a distinct division of Temnoclieihis ; bnt the near 
relationship between PleuronauHlus and Trematodiscus [Coelonau- 
ttlus] on the one hand, and the distinctness of these two genera 
from the coexisting Temnocheilus-stoek [Stammes] on the other 
hand, determined me to introduce PleuronavtiJus as a distinct 
genus. 
Fig. 22. 
PleuronantUus Mosis. — a, front view, showing the siphuncle and the internal or 
dorsal lobe below it ; h, lateral view, showing the ornaments of the test and 
the perforated umbilicus. Eeduced one-half from Mojsisovics’s figures in 
Abanhdl. d. k.-k. geol. EeichsausL. Band x. 1882, pi. Ixxxv. W. 3«, 2>b. This 
species is not in the collection, but the figures are given to illustrate the 
genus. 
“ The near relationship between Trematodiscus and PleuronantUus 
can easily be made out. First, the variation in certain species of 
Carboniferous Trematodiscus must be pointed out ; this consists in 
the development in advanced age of a full, rounded shell, free from 
sculpture {^Nautilus globatus, Sow., e. g.]. The union of the typical 
Trematodiscus sculpture with the lateral ribs of PleuronauHlus is 
observable in Gyroceras tessellatum^ de Koninck, G. hinodosum, 
Sandberger, G. costatum^ Goldfuss, Cyrtoceras ruyosum, Fleming, as 
well as in Nautilus nodoso-carinatus^ F. E,oemer\ of the Coal- 
Measures. Secondly, the characters presented by PleuronantUus 
marmolatce (in which the Trematodiscus sculpture passes directly 
into the PleuronantUus sculpture), added to the appearance in other 
Pleuronautili of several nodose-ridged species {PleuronantUus tri- 
nodosus^ P. Mosis, P. Cornalice, P. Fischeri, &e.), which are apparently 
^ Geologic von Oberschlesien, Taf. viii. f. 19. 
