174 MOLLUSCA. CEPHALOPODA. 



suture. The lobes and saddles are similar on opposite 

 sides of the shell ; commonly there is first the external lobe 

 (fig. 50 I) on the external margin, then the superior and 

 inferior lateral lobes (1 1, 2 1), and other lobes known as 

 auxiliary lobes (al). The saddles are arranged in a similar 

 manner, we have the external saddle (s), the lateral saddles 

 (1 s, 2 s), and auxiliary saddles (as). 



The surface of the shell may be smooth or variously 

 ornamented with striae, ribs, tubercles or spines. 



There are two sub-orders (1) Nautiloidea, (2) Ammon- 

 oidea. 



SUB-OBDEE. NAUTILOIDEA. 



The form of the shell is variable ; it may be straight, 

 arched, or spiral. The sutures are usually simple, but 

 occasionally undulate or denticulate. The septal necks 

 are (except in the genera Bathmoceras and Nothoceras) 

 directed backwards. The position of the siphuncle is 

 variable ; its interior is frequently contracted by calcare- 

 ous deposits. 



Orthoceras. Shell straight, occasionally slightly 

 curved, conical, transverse section usually circular. Septa 

 simple, concave; the last chamber large; aperture not 

 contracted. Siphuncle usually slender, cylindrical, and 

 central, or sub-central, sometimes ex-central. Ornamen- 

 tation variable. Tremadoc Beds to Trias. 



Actinoceras. Similar to the preceding, but the 

 siphuncle is large and is inflated between the septa, and 

 contains in the interior a large amount of calcareous 

 deposit. Ordovician to Carboniferous Limestone. 



Gomphoceras (= Phragmoceras). Shell fusiform, 

 straight or slightly curved ; section circular, sometimes 

 oval; last chamber very long, aperture contracted, T-shaped. 



