606 MOLLUSCA phylum vi 



Gentroceras Hyatt. Devonian to Carboniferous. Tetragonoceras Whiteaves. 

 Devonian. 



Family 20. Pleuronautilidae Hyatt. 



More 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, but 

 becoming dorsad in later stages. 



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

 Anoploceras Hyatt. Trias. 



VII. KONINCKIOCERATIDA Hyatt. 



Nautilicones with biangular sections at an early stage of groivth, developing later 

 into modified trapezoidal outlines as in many of the Hercoceratida, but shells are smooth, 

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

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

 trigonal through elevation of the venter. Anmdar lobes present in most genera. 

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

 during the gerontic stage. 



Family 21. Koninckioceratidae Hyatt. 



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

 tetrago7ial sections, and finally into highly compressed volutions. All are smooth and 

 have marked umbilical saddles. Volutions vnth broad umbilical zones lohich become 

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



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

 Stenopoceras, Peripetoceras Hyatt. Permian. 



Family 22. Solenocheilidae Hyatt. 



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 groioth of the dorsum 

 being remarkably rapid. Shells smooth, but the aperture in the gerontic stage may develop 

 peculiar lateral projections, especially at the umbilical shoulders, which are usually very 

 prominent. Siphuncle sub-ventran. 



Aipoceras, Oncodoceras Hyatt ; Asymptoceras Kyckliolt ; Solenocheilus Meek. 

 Carl)oniferous. 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. 



VIII. DiGONIOCERATIDA Hyatt. 



Pririiitive forms constantly retain depressed volutions having a more or less biangular 

 or sub-trigonal section ; specialised shells repeat these stages in the young, but subsequently 

 become more involute, and the sections change to reniform, sub-quadrangular or sub- 

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

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



