254 OPPELIA, HAPLOCERAS. 



still needs revision, since no doubt some of the geologically 

 younger Arietites have been improperly included here. Another 

 point, which needs farther investigation, is the relation of many 

 forms to the group of JEg. angulatum, Schloth. 



Ninety-six species. 



HARPOCERAS ACTION, d'Orb. T. 108, figs. 627, 628. 

 " BIFRENS, Brug. T. 103, fig. 556. 



SERPENTINUM, Schl. T. 103, figs. 558, 559. 



Genus OPPELIA, Waagen. 



Shell with umbilicus usually narrow, external side either 

 rounded only on the body-chamber or on all the whorls. Sculp- 

 ture sickle-shaped, body-chamber frequently geniculate, never 

 carinate or angular, embracing one-half to two-thirds of a whorl ; 

 margin of aperture sickle-shaped or with ears, always with 

 rounded external lobes. Siphon stout with calcareous sheath. 

 Aptychus divided, calcareous, thick, folded (Apt. lamellosus) ; 

 muscles of attachment near the margin in the lower half of the 

 shell. Lobes moderately branched, siphonal mostly shorter 

 than the first lateral ; lobular bodies slender with almost parallel 

 edges; lateral lobes divided into two principal symmetrical 

 branches. 



Oppelia branches off in the lower Oolite with Opp. mbradiata 

 from Harpoceras; the last representatives, as far as we know, 

 appear in the upper Jura of Stfcamberg, where a considerable 

 number of different forms are found. 



Seventy-one species. 



OPPELIA SUBRADIATA, Sowb. T. 110, fig. 650. 



Genus HAPLOCERAS, Zittel. 



The genus Haploceras was established by Zittel for a group 

 allied to Oppelia from the middle and upper Jurassic, which is 

 characterized by very feeble or no sculpture ; also some creta- 

 ceous forms, as Hapl. Grusanum are placed here ; and with 

 them forms very pronounced wedge- or chisel-shaped in section, 

 as Hapl. belus ; finally, species with quite sharp external sides, 

 as Hapl. nisus, Orb. 



In other Jurassic species of Haploceras, there is gradually 

 developed a transverse sculpture, which is confined to the ex- 



