576 MOLLUSCA SUB-KINGDOM VI 
median lateral line, and sometimes bifurcated externally. Nodes never present, 
although prominent crescentic ridges may arise on the sides through confluence of 
costae. The latter are straight in primitive Catulloceras, which resembles Caloceras 

Fic. 1200. 
Fic. 1199. Lioceras opalinum, Rein. sp. Lower 
Hildoceras bifrons, Brug. sp. Upper Lias; Whitby, Dogger; Teufelsloch, near Boll, Wiirtem- 
Yorkshire. berg. 
in aspect. Discoidal forms often both keeled and channeled on the venter, and 
sometimes have broad furrows on the sides. Specialised involute shells have solid 
keels, but usually no channels, and lateral 
Mags zones often become smooth. Sutures com- 
4 paratively simple, and in discoidal forms 
similar to those of Arnioceras, but more 
complex in highly involute forms. 
_  Hildoceras (Fig. 1199), Lioceras (Fig. 
1200), Grammoceras, Hyatt; Catulloceras, 
Gemm. ; Dumortieria, Haug (Fig. 1201) ; 
Arieticeras, Seg.; Hyperlioceras, Grapho- 
ceras, Brasilia, Darellia, Buckm. Upper 
Lias and Inferior Oolite. 
Family 7. Poecilomorphidae, 
Costae in some species remain nodose on 
umbilical shoulders until a late stage, and 
bifurcate externally as in Oppelidae, but 
subsequently may become separated into 



Oo] . . . . . . 
ae 2201. 2 . single sigmoidal ridges like those of Hildo- 
nortieri Sp. r Lias ; sininger iutem- . . . > 
ery Bp. «Upper siias,, Heutingen, Warem: - conmiliae: Young similar to those of the 
berg. 
latter family, and sutures also in discoidal 
forms, but become more complex in involute shells. 
Poecilomorphus, Huddlestonia, Brodieia, Cosmogyria, Welschia, Buckman; Ludwigia, 
Bayle. Inferior Oolite. 
Family 8. Oppelidae. (Oppelinae, Haug.) Discoidal and highly involute shells 
with sutures, form, and markings in primitive species that show affinity with Hildo- 
ceran stock, and apparent derivation from Poecilomorphus through typical Oecotraustes. 
Costae highly flexed and sometimes fused, but no well-marked lateral channels as in 
hollow-keeled groups. Venter often truncated and sides flattened, except in primitive 
