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, Sow. PI. I. 



— Bonnardii, d'Orb. PI. XI. 



— Crossii, Wright. PI. X. 



— obtnsus, Sow. PI. XXI. 



— semicostatus, Simp. PI. I. 



— bisulcatus, Brug. PI. III. 



— raricostatus, Ziet. PI. VII. 



Arietites rotiformis, Sow. PI. V. 



— Sauzeanus, rf'OrS. PI. VIII. 



— stellaris, Sow. PL XXII. 



• — Scipionianus, d'Orb. PI, XUI. 



— Conybeari, Sow. PL II. 



— Brooki, Sow. PL VI. 



— Turneri, Sow. PL XII. 



Genus Harpoceras, Waciff. — 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 ^"'- ^^^•" 

 than the principal, and passing inwards into several auxiliaries. 



The Aptyclius 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 : 



-Harpoceras serpenti- 

 num. Reinecke. 



Middle Lias. 



Harpoceras, sp. 



— Normanianum, d'Orb. 



Harpoceras serpentinum. Rein. 



— Levisoni, Simp. 



— bifrons, Briig. 



— falciferum. Sow. 



— Lythense, Young. 



— ovatum, Young. 



— variabile, d' Orb. 



Harpoceras impendens, Yovng and Bird. 

 — arietiforme, 0pp. 



Upper Lias. 



Harpoceras insigne, Sehilhl. 



— opalinum, Rein. 



— radians, Rein. 



— striatulum, Sow. 



— Aalense, Ziet. 



— Thouarsense, d'Oib. 



— priaiordiale, Schloth. 



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