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 



Hililocerns bifrons, Brug. sp. 

 Yorkshire. 



Upper Lias ; Whitby, 



FIG. 1200. 



Lioceras opallnum, Rein. 

 Dogger ; Te " 

 berg. 



p. Lower 

 sloch, near Boll, Win-tern - 



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 

 zones often become smooth. Sutures com- 

 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, Se,g. ; 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 

 single sigmoidal ridges like those of Hildo- 

 ceratidae. Young similar to those of tin- 

 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 shell- 

 with sutures, form, and markings in primitive species that show affinity with Hildo- 

 ceran stock, and apparent derivation from Poecilomorphus through typical Oecotrauste*. 

 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 



Dumortii ria, 

 berg. 



FIG. 1201. 

 Upper Lias ; Heiningen, Win-ten 



