HAPLOCERAS. 255 



ternal side of the body-chamber (Hapl. jungens, Neum., carach- 

 theis, Zeuschner). 



In certain upper Jurassic forms, which are allied to Hapl. 

 carachtheis, the sculpture gradually passes from the external 

 side over to the flanks in feebly undulating ribs, as is shown in 

 Hapl. cristiferum, Zitt. ; better developed in Hapl. wohleri, 

 Opp. ; and this feature is repeated in Hapl. difficile, Orb., Cleon, 

 Orb., bicur-vatum, Leym. 



Finally, species of Haploceras appear which are distributed 

 in the Cretaceous, with constrictions reaching forwards (Hapl. 

 Beudanli, Parraudieri), a peculiarity with which I have not met 

 in any Jurassic form ; the inner whorls here serve as sure guides, 

 aside from the agreement of the lobular markings, since they 

 represent a typical Haploceras with entirely smooth whorls. 

 With these furrows a sickle-shaped undulating radial sculpture 

 is gradually combined, and a group of forms results, of which 

 the principal type is Hapl. planulatum, Sow. 



In spite of this great manifoldness, it is very easy to distin- 

 guish the representatives of Haploceras from strata which are 

 lower than the Turonian and downwards, by their whole habitus 

 and lobes, yet nothing is more difficult to express in words. 



The number of lobes in Haploceras varies, since besides the 

 siphonal lobe and the two laterals two to four auxiliaries are 

 present ; the lateral lobes are never symmetrically divided (a 

 difference from Lytoceras}, and never present the characteristic 

 rounding of the saddle lobes of Phylloceras ; in the forms from 

 the Neocomian the lobes are not yet very complicated, but later 

 are much branched, with slender stems ; the stems of lobes 

 mostly broader than those of the saddles, the first lateral not 

 strikingly larger than the second. 



If we compare the lobes of other forms, Schloenbachia, Amal- 

 theus, Phylloceras, Lytoceras and Acanthoceras are excluded 

 from consideration; a difficulty can only arise in regard to 

 Hoplites, which certain forms resemble in their lobular structure. 

 Only here the width of the lobes and bodies of the saddles will 

 seldom allow of a remaining doubt, since the latter are, as a 

 rule, broader than in the first, the strong development of the 

 external saddle, the striking difference in size between the two 

 laterals, finally the broader, better rounded forms of the lobes 



