254 OPPELIA, HAPLOCERAS. 



still needs revision, since no doubt some of the geologicall}' 

 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. lamellosui^) ; 

 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 s^- mmetrical 

 branches. 



Oppelia branches off in the lower Oolite with Op)p. mbradiata 

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

 appear in the upper Jura of Stramberg, where a considerable 

 number of different forms are found. 



Sevent3^-one species. 

 Oppelia subradiata, Sowb. T. 110, fig. 650. 



Genus HAPLOCERAS, Zittel. 



The genus Haplocera^ was established b}^ 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. Grusanxim 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 Haploccras., there is gradual^ 

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



