THE CEPHALOPODA. 197 
V 
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 umbilicus. Found in the Lower 
Chalk, Gault, and Neocomian Formations. HopUies 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. Capri€orni, von Bach, See p. 188. 
YiG.?,\.—Hoplites Martina, A'Orh. Fig. 82. Fig. 83. 
Phylloceras helerophyllum, Sow. 
g. Species with the siphonal area convex. 
16th Section. Heteeophylli, 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. 
JPhylloceras heterophyllum (figs. 82, 83), from the Upper Lias, is typical of this group. 
17th Section. Ligati, d' Orhigny. Shell in general compressed, smooth, or a httle 
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 
obliquely backwards towards the umbiUcus. The saddles very much divided and never 
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