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Genus Asprpoceras, Zittel—The form of the shell in this 
genus is very variable: sometimes it is flat and discoidal with 
a wide umbilicus, or it is large, inflated, and highly involute. 
The siphonal or ventral area is rounded or flattened, the sides 
are adorned with fine ribs, and the sculpture consists of two 
rows of tubercles developed at intervals among the finer folds of 
the shell in its early age, which appear to disappear and are 
undeveloped in later years. The lobe-line ramifications are 
simple in Aspid. perarmatum, Sow. The siphonal lobe is large 
with symmetrical branches, the principal lateral is large, and 
composed of numerous unequal parts, and the lower lateral is 
much smaller. The body of the lobes and saddles is broad, and 
the lobes are much slit up into branches. Aspid. longispinum, 
Sow., from the Oxfordian of Weymouth, has a thick, discoidal, 
smooth shell, with two concentric rows of short spines growing 
upon the sides of the whorls, which are few and more than one 
half involute (fig. 82.) This forms a very good type of the 
group. The large section of the Perarmati, with a double 
row of tubercles on the sides of the whorls, appears to want 
auxiliary lobes. In the group which has Aspid. altenense, 
p’Ors., for its type, from the corallian, one row of tubercles 
grows near the umbilicus, and the sides and ventral area are 
encircled by small fine undivided lines of growth. 
Genus PExroceras, Waag.—Shell flat, discoidal, with a very 
large umbilicus; whorls with straight ribs, which, in adult 
specimens, are mostly provided with two or three rows of spines, 
siphonal side more or less flattened, or even excavated, the ribs 
passing over or disappearing before they reach it. Inner whorls 
with strong, sharp, mostly dichotomate ribs, sometimes undivided, 
outer whorls with strong, blunt, bi-tuberculous ribs, which pass 
over the siphonal area. Lobe-line highly ramified; siphonal 
lobe large, with symmetrical divergent branches ; superior 
lateral widely separated from the siphonal, and forming a broad 
external saddle; lower lateral small, and imperfectly developed, 
or even wanting; sometimes the ribs are in part replaced by 
spines; amount of involution small; length of body-chamber 
unknown; mouth-border with large lateral auricles developed 
