CEPHALOPODA. 69 



Genus 10.— AMMONITES.— ZamarcA. 



Generic Character. — Shell discoid, spiral ; volutions con- 

 tiguous, wholly visible ; inner parietes articulated by sinuous 

 sutures ; septa transverse, lobed at the circumference, and 

 without siphon at the disk, but perforated by a marginal tube 

 near the margin. 



Ammonites armatus — The Armed Ammonites. Plate 

 X. fig. 7. Subcarinated, volutions depressed at their inner 

 margins ; with strong transverse strise and clavate 'ribs. 

 Fossil. 



FAMILY II. NAUTILACEA. 



Shell discoid, with a central spire, and short cells, which 

 do not extend from the centre to the circumference. 



Genus 11.— NAUTILUS Linnceus. 



Generic Character. — Shell discoid, spiral, multilocular, 

 with simple parietes; volutions contiguous, the last or body 

 one enveloping the rest; the cells numerous, septa trans- 

 verse, externally concave, perforated in the disk by a tube ; 

 margins entire. 



Nautilus umhilicatus The Umbilicated Nautilus. 



Plate X. fig. 8. Suborbicular ; the centre of the volutions 

 umbilicated on both sides; aperture round, heart-shaped 

 pale fawn-coloured, with chestnut undulated ti'ansverse 

 clouds. Six inches long. Inhabits the Indian Ocean. 



Genus 12._NUMMULITES._Za»«arc/l. 



Generic Character Shell lenticular, attenuated at the 



margins ; spire internal, discoid, multilocular, covered over 

 by several plates; the outer layers complicated, produced, ex- 

 tending and uniting on each side to the central disks ; cells 

 very numerous, small, alternate, and formed by transverse 

 imperforate septa. 



