SUB-ORDER D ORTHOCHOANITES 525 
the venter flattened or concave, and dorsum remaining convex until a late stage. 
Nautilicones have a persistent conver centran area in the impressed zone. No annular 
lobes known, 
Centroceras, Hyatt. Devonian to Carboniferous. Tetragonoceras, Whiteaves. 
Devonian, 
Family 20, Pleuronautilidae. 
More or less discoidal nautilicones with stout volutions and large umbilical perfor- 
ations ; the young, especially in primitive species, remaining cyrtoceracones until a late 
stage. More specialised shells are costated and tuberculated on the sides. Sutwres have 
annular lobes except in Pselioceras. Siphuncle ventrad of centre in the young, but 
becoming dorsad in later stages. 
Pselioceras, Hyatt. Permian.  Plewronautilus, Mojs.; Encoiloceras, gen. nov. 
>} y b) 2 2) AS) 
Type #. (Plewr.) superbus, Mojs. sp.; Enoploceras, gen. nov. Type FE. (Naut.) Wulfeni 
J o] z) ko) I ? 
Mojs. sp.; Anoploceras, gen. noy.. Type A. (Pleur.) ampezzanus, Mojs. sp. Trias. 
VII. KONINCKOCERATIDA. 
Nautilicones with biangular sections at an early stage of growth, developing later 
into modified trapezoidal outlines as in many of the Hercoceratida, but shells wre smooth, 
and the trapezoidal form as a rule evolves during the phylogeny into quadrangular, a 
jinally into involute coils with compressed sections, or may become simply more or less 
trigonal through elevation of the venter. Annular lobes present in most genera. 
Aperture constantly open, and in some forms remarkable lateral projections are developed 
during the gerontic stage. 
Family 21. Koninckoceratidae. 
Shells of primitive forms similar to Temnocheilus, but leading into those with 
tetragonal sections, and finally into highly compressed volutions. All are smooth and 
have marked umbilical saddles, WVolutions with broad wmbilical zones which become 
lateral in the more involute species. Stphuncle ventrad of centre. 
Koninckoceras, Domatoceras, Hyatt. Carboniferous. Potoceras, Hyatt. Devonian (?). 
Stenopoceras, Peripetoceras, Hyatt. Permian. 
Family 2 22. Solenocheilidae. 
Compressed elliptical in section during early stages, but full-grown of primitive forms 
and young of specialised derivatives have a more or less trigonal section in neanic stage. 
Later this stock evolves shells with volutions having depressed elliptical or broadly hemi- 
_ spherical outlines. Sutures generally have large ventral saddles, and saddles on the 
umbilical shoulders. Umbilical zone very broad, the increase by growth of the dorsum 
being remarkably rapid. Shells smooth, but aperture in gerontic stage may develop 
peculiar lateral projections, especiaily at the umbilical shoulders, which are usually very 
prominent. Siphuncle sub-ventran. 
Aipoceras, Oncodoceras, Hyatt; Asymptoceras, Ryckholt; Solenocheilus, Meek. 
Carboniferous. Pteronautilus, Meek. Permian. 
Acanthonautilus, Foord. Nautilicones with sub-hemispherical volutions, the 
dorsum flattened or concave. Aperture developing laterally into two projecting spines 
at the umbilical shoulders. Carboniferous. 
