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growth. Runzeln present; through the whole length of the 
body-chamber up to the mouth-border there rises upon the 
inner side of the shell, upon its convex part, a median ridge 
(normal line.) The whorl is contracted near the mouth. The 
lobe-line is formed by a few pairs of small incised saddles, which 
sometimes alternate with single saddles. The radial lines of 
growth on the shell, the median ridge on the ventral surface, 
and the peculiar arrangement of the lobe-line distinguish 
Didymites from Arcestes. All are collected from the Norische 
Stufe of the Trias, near Hallstatt. 
Genus Lonrrns, v. Mojs.—In the general form and length of 
the body-chamber it agrees with Arcestes and Didymites. Shell 
with transverse folds, which are abundantly intersected with 
longitudinal lines. The last whorl developes a remarkable 
inflation, and subsequent contraction, and has the mouth- 
border much thickened, and the umbilicus occluded. Most of 
the species are found in the Triassic beds. 
Genus Prycuitss, v. Mojs.—In its general form and length of 
the chamber it resembles Arcestes, but differs from that group 
in the structure of the lobes. 
Genus Pryacoceras, v. Mojs.—Shell small, with a high mouth- 
opening, surface smooth, rarely with any elevations thereon ; 
body-chamber from one half to two thirds of a whorl long. 
Mouth-border with short lappet-formed processes on the ventral 
side. Lobe-line distinguished by the presence of an additional 
external lobe. The species are all Triassic. Hx. Pinacoeras 
imperator, Hav., Pinacoc. Daonacum, Moss. 
Genus Saceceras, v. Mojs.—In the form of the shell and 
length of the body-chamber this genus is nearly related to 
Pinacoceras. It is distinguished from it by the structure of the 
Runzelschicht, the form of the lobes, and the direction of the 
lines of growth on the concave part of the shell. The Runzel- 
schicht is granular, as in Nautilus, and does not consist of long 
lines and threads, as in other Arcestide. The saddles are small, 
rounded, or linguiform ; the lobes symmetrical, through simple 
conical indentations, single or divided; there are three lobe 
groups, as in Pinacoceras, three species belong to the Permian 
