189 
I Family.—Axcestiw#, E. v. Mojsisovics. 
Shell smooth or provided with transverse folds, ribs or 
longitudinal lines. Runzelschicht (wrinkled layer)* present in 
the older geological forms, the lobes consist mostly of short, 
abruptly interrupted lines, seldom (only in Sageceras) granular. 
Mantle impression in the forms from the Trias, with or without 
contraction of the mouth, always on the mould of the body- 
chamber, seldom visible on the cellular portion of the shell. 
Horny Aptychus probably present in Arcestes, certainly in 
Amaltheus, for the other forms doubtful. 
Genus ArcustEs, Suess.—Shell in general smooth, seldom with 
longitudinal lines ; body-chamber long, from one to one and a 
half whorls. Spiral growth slow, strongly involute, mouth- 
opening contracted, either by an inversion of the shell, or 
through a thickening of its inner lamina around the border. 
Lobes strongly incised, so that the saddles consist of a small 
stem, with numerous approximated horizontal branches, from 
which other bifurcations proceed. The umbilicus is often 
closed by shelly eversion. 
Dr. Ep. v. Mossisovics divides the genus into the following 
groups :—Extra-labiati, Sub-labiati, Bi-carinati, Coloni, Intus- 
labiati, Galeati, and Sub-umbilicati, all of them characterised 
by more or less modified body-chambers of the full-grown 
individuals, and by a contraction of the umbilicus frequently 
entirely occluded by a callosity. All these forms are accurately 
figured from perfect specimens in his splendid monograph. 
The typical Arcestes belong to the Trias. The species number 
about 130, and the Muschelkalk contains some highly developed 
forms. I cite three well-known species as types of this genus: 
Arcestes, sub-wmbilicatus, Bronn, Arcestes Gaytani, Klipstein, 
Arcestes Johannis Austric, Klipstein. 
Genus Dipymrres, v. Mojs.—General form and length of 
chamber as in the preceding; shell with well-defined lines of 
* This word has no equivalent in the English language, but it clearly 
means the wrinkled marginal layer of the mantle of the mollusc that formed 
the lobe lines of the septa, and which imparted such an important character 
to the shells of the Ammonitide. 
