18 MOLLUSCA. CEPHALOPODA. 



FAMILY I. AMMONACEA. 



Septa sinuous, lobed and cut at the margin, meeting together 

 upon the inner wall of the shell, and articulated by jagged 



sutures. 



Gen. 6. BACUXITES, Lam. 



Shell straight, cylindrical, sometimes a little compressed, slightly 

 conical ; the walls articulated by sinuous sutures ; septa trans- 

 verse, close, imperforate, lobed and laciniate at the margin. 



The species of this genus are all fossil. 



Gen. 7. TURRILITES, Lam. 



Shell spiral, turreted ; the whorls contiguous, and all externally 

 perceptible ; the walls articulated with sinuous sutures ; septa 

 transverse, lobed and laciniated at the margin ; aperture round- 

 ed. 



The species of this genus are fossil. In Britain they have been found in chalk marl. 



Gen. 8. AMMONOCERAS, Lam. 



Shell horned-shaped, arcuate, subsemicircular ; the walls arti- 

 culated with sinuous, laciniate, branched sutures; septa 

 transverse, sinous, imperforate ; their margins lobed, laciniate; 

 a marginal tube or syphon, not perforating the septa. 



The species of this genus are all fossil. 



Gen. 9. ORBULITES, Lam. 



Shell subdisciform, spiral, with the turns contiguous, the last 

 covering the rest; the inner walls articulated by sinuous 

 sutures ; septa transverse, lobed at their circumference, and 

 perforated by a marginal tube. 



The species of this genus are all fossil. Those found in Britain are from the 

 carboniferous limestone, oolite, lias, and chalk marl. See Fleming's Brit. An, 248. 



Gen. 10. AMMONITES, Lam. 



Shell disciform, spiral, the turns contiguous, and all visible ; the 

 inner walls articulated with sinuous sutures ; septa transverse, 

 incised and lobed at the margin, imperforate in their disk, 

 but perforated with a sort of marginal tube. (Fossil.) 



The Ammonites are among the most remarkable and best known of the fossil 

 shells, being of frequent occurrence in most parts of Europe. They are found in 

 England in the London clay, oolite, lias, chalk marl, and green sand. For the 

 species see Fleming, 240. Some of them are upwards of two feet in diameter. 



FAMILY II. NAUTILACEA. 



Shell disciform, with a central spire, and short cells, which do 

 not extend from the centre to the circumference. 



Gen. 11. NAUTILUS, Lin. 



Shell disciform, spiral, multilocular, with simple walls ; turns 

 contiguous, the last covering the rest ; transverse septa, con- 

 cave externally, perforated in the disk ; the margins entire. 



