606 MOLLUSCA phylum vi 



Gentroceras Hyatt. Devonian to Carboniferous. Tetragonoceras Whiteaves. 

 Devonian. 



Family 20. Pleuronautilidae Hyatt. 



Mo.re or less discoidal nautilicones with stout volutions and large umhilical perfora- 

 tions ; the young, especially in primitive species, remaining cyrtoceracones until a late 

 stage. More specialised shells are costated and tuherculated on the sides. Sutures have 

 annular lobes except in Pselioceras. Siphuncle ventrad of centre in the young, hut 

 hecoming dorsad in later stages. 



Pselioceras Hyatt. Permian. Pleuronautilus Mojs. ; Encoiloceras, Enoploceras, 

 Änoploceras Hyatt. Trias. 



VII. KONINCKIOCERATIDA Hyatt. 



Nautilicones with hiangular sections at an early stage of growth, developing later 

 into modißed trapezoidal outlines as in many of the Hercoceratida, hut shells are smooth, 

 and the trapezoidal form as a rule evolves during the phylogeny into quadrangular, and 

 finally into involute coils with compressed sections, or may become simply more or less 

 trigonal through elevation of the venter. Annular lohes present in most genera. 

 Aperture constantly open, and in some forms remarJcahle lateral projections are developed 

 during the gerontic 



Family 21. Koninckioceratidae Hyatt. 



Shells of primitive forms similar to Temnocheilus, hut leading into those with 

 tetragonal sections, and finally into highly compressed volutions. All are smooth and 

 have marked umhilical saddles. Volutions vnth hroad umhilical zones which hecome 

 lateral in the more involute species. Siphuncle ventrad of centre. 



KonincMoceras, Domatoceras Hyatt. Carboniferous. Potoceras Hyatt. Devonian (?). 

 Stenopoceras, Peripetoceras Hyatt. Permian. 



Family 22. Solenocheilidae Hyatt. 



Compressed elliptical in section during early stages, hut 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 hroadly hemi- 

 spherical outlines. Sutures generally have large ventral saddles, and saddles on the 

 umhilical Shoulders. Umhilical zone very hroad, the increase hy growth of the dorsum 

 heing remarkahly rapid. Shells smooth, hut the aperture in the gerontic stage may develop 

 peculiar lateral projections, especially at the umhilical Shoulders, which are usually very 

 prominent. Siphuncle suh-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 

 umhilical Shoulders. Carboniferous. 



VIII. DiGONiocE RATIDA Hyatt. 



Primitive forms constantly retain depressed volutions having a more or less hiangular 

 or suh-trigonal section ; specialised shells repeat these stages in the young, hut suhsequently 

 hecome more involute, and the sections change to reniform, suh-quadrangular or suh- 

 elliptical. Shells smooth except in the single genus Gymatoceras. Aperture simple and 

 open at all stages ; gerontic living chamher only slightly contracted. 



