MODERN CLASSIFICATION. 
249 
The genus Arietites, as long ago remarked by Leopold von Buch, is very character- 
istic of the Lower Lias, in which we find the following species : 
Arietites Bucklandi, Sotv. PI. I. 
— Bonnardii, d'Orb. PI. XI. 
— Crossii, Wi'ight. PI. X. 
— obtusus, Soiv. PI, XXI. 
— semicostatus, Simp. PL I. 
bisuleatus, Brug. PL III. 
— raricostatus, Ziet. PL VII. 
Arietites rotifonnis, Soio. PL V. 
— Sauzeanus, <Z'0?-6. PL VIII. 
— stellaris, Sow. PL XXII. 
— Scipionianus, d'Orb. PL XIII. 
— Conybeari, Sow. PL II. 
— Brooki, Sow. PL VI. 
— Turner], Sow. PL XII. 
Genus Harpoceras, Waag. — The external outline of the 
shell is of various forms, the ventral area always keeled with a 
round or angular border ; the sides of the whorls ornamented 
with elegant falciform ribs of various degrees of thickness. 
Mouth-border sickle-shaped, with lateral auricles, and a long, 
pointed, ventral process (fig. 155). Body-chamber from one 
half to two thirds of a whorl in length. Suture-line moderately 
ramified. Siphonal lobe short and bifurcate, with divergent 
branches, and always shorter than the principal lateral lobe, 
which is largely ramified. The second lateral lobe much smaller 155.— ^^ar^ocera* serpenti- 
o J num., Keinecke. 
than the principal, and passing inwards into several auxiliaries. 
The A'piyclius bivalved, thin, and calcareous, with a shelly covering and several 
longitudinal folds, as seen in Harp. Levisoni, Simp. 
Harpoceras first appears in the Middle Lias, in the zone of Aeg. Jamesoni ; it 
attains its highest development in the Upper Lias, and is found in the Lower and 
Middle Jurassic rocks, disappearing in the Upper Kimmeridgian with Harp. Zio, 0pp., 
collected from the zone of Pterocera Oceani, at Mahringen, near Ulm, Wiirttemberg. 
The distribution is as follows : 
Middle Lias. 
Harpoceras, sp. 
— Normanianum, cfOrb. 
Harpoceras serpentinum, Rein. 
— Levisoni, Simp. 
— bifrons, Briig. 
— falciferum, Sow. 
— Lythense, Young. 
— ovatum, Young. 
— variabile, c?' 
Harpoceras impendens, Young and Bird. 
— arietiforme, 0pp. 
Upper Lias. 
Harpoceras insigne, Sclmhl. 
— opalimim, Rein. 
— radians. Rein. 
— striatulum. Sow. 
— Aalense, Ziet. 
— Tliouarseuse, d'Orb. 
— primordiale, Schloth. 
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