144 



THE GEOLOGIST. 



guishable from those of the Jurassic rocks, by the form of 

 the saddle. 



This Group, which was not formed by Von Buch, as he 

 was only acquainted with a single species, was nevertheless, 

 clearly foreseen by him.* It is at present as numerous as 

 many others. 



17th Group. The Ligati, d'Orb, — *SM/ compressed, gene- 

 rally smooth or slightly undulating, frequently marked at 

 certain distances, with grooves or ribs, indicating the position 

 of former successive margins. Back convex, sometimes 

 slightly compressed. Septa composed of lobes formed of 

 unequal parts, and saddles generally equal. The dorsal lobe 

 shorter than the superior lateral one ; the last auxiliary lobes 

 often oblique posteriorly, towards the umbilicus ; the saddles, 

 much divided, without foliations. All the species are from 

 the Cretaceous rocks [A. ligatus, intermedins, cassida, dispar, 

 Jleocisidcatus. Emerici, Belus, Rogerianus, impressus, inornatus, 

 Carter oni, Grasianus, Cesticulatus, raresulcatus, from the Neo- 

 comien; A. Beaudanti, latidorsatus, Parandieri, Clementinus 

 Mayorianus, Dupinianus, versicostatus, from the Gault or 

 Lower Green Sand ; A, Levjesensis, peramplus, Prosperianus^ 

 from the Upper Green Sand, or Chloritic Chalk. 



18th Group. The Planulati, Buch. — Shell discoidal, com- 

 pressed, whorls more or less cylindrical, ornamented with 

 serrated strise or ribs separating, about the middle or two- 

 thirds of the sides, into several branches, without any tu- 

 bercule at the point of junction. Back round. Septa formed 

 of lobes, always divided into unequal parts, and of sad- 

 dles generally formed of equal parts. The dorsal lobe 

 either longer or shorter than the superior lateral lobe ; 

 the auxiliary lobes very oblique posteriorly, towards the um- 

 bilicus. This group is peculiar to the Jurassic or Oolitic 

 formations {A. communis, polyplocus, plicatilis, annulatus), 

 * Annales rles Sciences Natnrelles, 1830, tome xxix. p 27. 



