THE CEPHALOPODA. 



197 



nearly square. Siphonal area much narrower than the sides. Ribs elevated, passing 

 alternately across the area from the one side to the other. Septa composed of lobes 

 formed of single parts, and of saddles often in pairs. Siphonal lobe shorter than the 

 superior lateral ; the auxiliary lobes oblique towards the umbihcus. Found in the Lower 

 Chalk, Gault, and Neocomian Formations. Hoplites Martinii, d'Orb. (fig. 81), from the 

 Neocomian, or Lower Greensand of the Isle of Wight, is a fair type of this group. 



15th Section. Capricorni, von BticJi. See p. \[ 



iJWlT'fW, 



Tig. 81. — Hoplites Martinii, d'Orb. 



Fig. 82. Fig. 83. 



Phylloceras heterophyllum^ Sow. 



ff. Species ivith the siphonal area convex. 



16th Section. Heterophylli, d'Orhigny. Shell compressed, formed of whorls 

 almost completely involute, rarely appearing in the umbilicus. The sides are smooth, 

 slightly striated, or furrowed with fine lines. Siphonal area narrow and very convex. Septa 

 symmetrical, divided into a great number of lobes, very much ramified, formed of single 

 parts, and of saddles often in pairs. Siphonal lobe always shorter than the upper 

 lateral. The great number of ramifications of the lobes produces a most complicated, 

 foliated structure on the siphonal area and sides of the casts of this section. 



Phylloceras heterojihylhim (figs. 82, 83), from the Upper Lias, is typical of this group. 



17th Section. Ligati, d' Orhigny. Shell in general compressed, smooth, or a little 

 undulated, and usually marked with grooves or ribs at regular distances apart, indicating 

 the former position of labial appendages. The siphonal area convex, sometimes a little 

 compressed. Septa composed of lobes formed of single parts and of saddles, often in pairs. 

 The siphonal lobe shorter than the superior lateral. The last auxiliary lobes often directed 

 obhquely backwards towards the umbilicus. The saddles very much divided and never 



2G 



