MOLLUSCA. CEPHALOPODA 



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which bear tubercles or spines arranged in spiral rows ; at the 

 external margin is a groove ; umbilicus generally small. Body- 

 chamber short. Sutures simple, lobes and saddles toothed. Trias. 

 Ex. T. aon. 



Arcestes. Shell smooth or striated, nearly globular, with 

 thick whorls ; umbilicus small or absent ; aperture without lateral 

 projections ; body-chamber very long. Lobes and saddles numerous 

 and foliaceous, arranged in a 

 straight row and gradually de- 

 creasing in size from the external 

 to the internal margin ; there are 

 two lateral lobes and many 

 auxiliary lobes; saddles with 

 narrow stems and fine branches. 

 Trias. Ex. A. intuslabiatus. 



Phylloceras (figs. 112,113). 

 Shell smooth or with fine striae or 

 gentle folds, never with tubercles ; 

 external margin rounded ; umbili- 

 cus very small or absent. Saddles 

 and lobes numerous ; saddles 

 divided, the extremities being 

 rounded ; auxiliary lobes numerous 

 P. heterophyllum, Upper Lias. 



Fig. 112. Phylloceras heterophyl- 

 lum, from the Lias. A part 

 of the shell has been removed 

 to expose the sutures, x |. 



Jurassic to Cretaceous. Ex. 



Fig. 113. Suture line of Phylloceras heterophyllum, from the Lias. The 

 arrow indicates the position of the siphuncle and points towards the 

 aperture of the shell. (From Woodward.) Natural size. 



Lytoceras (fig. 114). Shell ornamented with transverse ribs, 

 and often with laminar projections (varices) placed at intervals. 

 Whorls rounded, and only slightly or not at all embracing ; aperture 



w. p. 



18 



