MOLLUSCA — CEPHALOPODS 261 



tion as regards the anatomy of the living animal, such as the 

 number of gills and kidneys, are classed with this order because 

 their shells agree closely with that of Nautilus. 



Cephalopods have been the strongest competitors of the ver- 

 tebrates in the oceans from Silurian time to the present. 

 Sections through the living chambers of different Paleozoic 

 species show scales or other armor of the fish then living, indicat- 

 ing that such animals formed part of their food. 



This order includes the two great groups Nautiloidea and 

 Ammonoidea ; the former have a non-calcareous and the latter 

 a large calcareous initial chamber, the protoconch (Fig. 116, pr.). 

 The siphuncle in the nautiloids varies in position from dorsal to 

 ventral, being often centrally placed ; in ammonoids it is, 

 with the exception of one group, ventral in position, i.e. upon 

 the outer edge of the coil. The curved growth lines of the 

 hyponomic sinus almost invariably indicate the ventral side. 

 In general the size of the siphuncle decreases from the nautiloids, 

 through the ammonoids and belemnoids, to the sepioids, where 

 it is a mere rudiment, distinguished with difficulty. This same 

 relative decrease in the size of the siphuncle is seen in the devel- 

 opment of an individual ammonoid from youth to maturity. 

 Bactrites with its straight shell, simple sutures, ventral, but sub- 

 marginal, siphuncle, peculiar collars and large calcareous proto- 

 conch may be transitional between the two groups, the Nauti- 

 loidea and the Ammonoidea. 



Sub-order a, Nautiloidea. — Shells straight, curved, or coiled. 

 Sutures simple, i.e. straight or undulated, never acutely angular. 

 Living chamber usually with aperture notched by a hyponomic 

 sinus for the passage of the funnel. Siphuncle usually central 

 or dorsal, i.e. on inner margin of whorl, rarely ventral. Shells 

 smooth or with simple ridges or nodes, never complexly orna- 

 mented. 



Nautiloids are first known from the Cambrian rocks ; they 

 reach their maximum development in the Silurian and decline 

 to the Triassic ; since that period their retrogression has been 



