244 Lamarck's Genera of Shells. 



each extending from the centre to the circumference. Their aper- 

 ture is the chief character which distinguishes them from the len- 

 ticulites. 



Type. Placentula pulvinata *. 



No further description. PI. vi. Fig. 224. 2 Species, both 

 recent. 



6th Family. 

 Nautilacea. (6 genera.) 

 Shell discoidal, spire central, chambers short, not extending 

 from the centre to the circumference. 



The nautilacea differ widely from the radiolata, in having the 

 spire composed of several whorls, wherefore the chambers cannot 

 extend from the centre to the circumference : their spire is also 

 complete, which that of the radiolata never is. 



1. Discorbitest. 



Shell discoidal, spiral, multilocular ; sides simple. All the 

 whorls visible, naked, contiguous to one another. Septa transverse, 

 frequent, imperforate. 



The discorbites differ from the nautili, by having all the whorls 

 of the spire visible, and no siphon: from rotalites, by the aperture 

 not inclining downwards towards the base, and the spire not rising 

 into a cone. 



One species. Discorbites vesicularis*. 



Shell discoidal ; whorls nodular at the chambers, subvesicular; 

 last chamber somewhat closed. Fossil, Grignon. PI. vi. Fig. 22.5. 



Note, by Lamarck. The Cornu ammonis vulgatissimum of Plancus, 

 (de Conch. Arimiu. p. 8. t. 1. f. 1.) must be referred to this genus. 



2. Siderolites 



Shell multilocular, discoidal ; whorls contiguous, not visible 

 externally ; disc convex on both sides, and loaded with tubercular 

 points; circumference bordered with unequal radiating lobes. 

 Septa transverse, imperforate. Aperture distinct, stiblateral. 



The siderolites are very small, star-shaped shells, with a sub- 



* Made like a cushion, or pillow. f From discus, a disk, and orbis, an orb. 

 J Vesicular. § From sidus, a star. 



