EVIDENCES— GEOLOGY 



133 



Octopoda 

 8 armed 

 Argonauta. Male 

 shell-less, 

 female with 

 external shell, 

 unchambered. 

 Octopus. Shell- 

 less. 



Belemnoidea 

 10 armed 

 Belemnites. Internal 



shell, partially 

 chambered, extinct. 

 Spirula. Internal 

 shell, wholly 

 chambered. 



Sepioidea 

 Internal vestigial 

 shell. Squids & 

 cuttlefish. 



Nautiloidea 

 Shell pri- 

 mitive with 



simple 

 sutures, not 

 highly ornamental. 



Mollusca, 



primitive radicle 



(Modified, after Lull) 



By comparing this diagram with the table it will be noted that 

 Mollusca of other groups occurred in the Cambrian. The earliest 

 Nautiloidea, derived at some unknown 

 point from the moUuscan stem, are the 

 oldest group of cephalopods, and appear 

 first in the Ordovician. From this stem 

 arise the more highly specialized Am- 

 monites, which first appear in the De- 

 vonian (Fig. 72). The Ammonites then 

 become extinct, while the nautiloids con- 

 tinue to the present, although with de- 

 creasing abundance and diversity. A 

 divergence of the ancestral stem gives 

 rise to three other forms, the Belem- 

 noidea, Octopoda, and Sepioidea. Of 

 the main branch, the belemnoids, many 

 fossils are found in the Jurassic and 

 Lower Cretaceous rocks, apparently de- 

 rived from another group during the 

 early Triassic. The squids, however, F^^- 72.-A shell oi 11 ete- 

 "^ 1 T • u-i iu roceras, a cephalopod 



appear first m the Jurassic, while the moll use. (From Lull, 



origin of the Octopoda is not indi- after Schuchert.) 



