MODERN CLASSIFICATION. 239 



which is seen on the mould of the bodj-chamber, but is seldom visible on the chambered 

 portion of the shell. Horny Aptychus probably present in Arcestes, certainly in Amal- 

 them, for the other forms doubtful. 



Genus Arcestes, Suess. — Shell in general smooth, seldom with longitudinal lines ; 

 body-chamber long, from one to one and a half whorls. Spiral increase moderate ; strongly 

 involute ; mouth-opening contracted, either by an eversion 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. Ed. v. Mojsisovics 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-unibilicatus, Bronn, Arcestes Gaytani, Klipstein, 

 Arcestes Joliannis- Austria, Klipstein. 



(?<?«?;s DiDYMiTEs, V. Mojs. — General form and length of chamber as in the preceding; 

 shell provided with folds and well-defined lines of growth ; through the whole length of 

 the body-chamber up to the mouth-border there rises upon the convexity of the 

 inner side of the shell 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 frequently 

 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 

 Bidymites from Arcestes. All are collected from the Noric stage or division (Norische 

 Stufe) of the Trias, near Hallstatt. 



Genus Lobites, 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 fine longitudinal lines. The 

 last whorl takes on frequently a shape different from the form of the 

 internal cast, and not unfrequently closes the umbilicus with a 

 callosity. Most of the species are found in the Triassic beds 

 (Carinthian division) with Lobites ellijMcus at Rothelsteins. I have 

 cited as a typical example (fig. 133) Lobites delj^hinoceplLolus, Hauer, 



from that locality. delpMnocephatus. 



' ' Das Gebirge urn Hallstatt,' 4to., with 70 plates, AVieii, 1875. 



