SI-H-ORDERD ORTHOCHOANITES 526 



the venter flattened or concave, mnl //-,/-.</'/,/ ///////////// eonvest /'/</// </ /"' 

 \,int/l/<;iiii'x have a persistent convex '////"// "/" /'// /// ////// .<.>../ rone, A'-/ " ////<//</ / 

 kiii > n- ,i, 



Centroceras, Hyatt. Devonian M < 'arl>oniferous. Tetmyonoceras, Whiteaves. 

 Devonian. 



Family 20. Pleuronautilidae. 



.}//<> or less (1i*-ni<liil nnutilicones vitl, *t not, volutions ami /,/,-,/, umbilical perfor- 

 ations ; the young, especially in primitive species, remain in <i <-m >'"> /<"< /,/..-,- ////// ,/ ///. 

 sto(/. More specialised shells are costated and tuberculated on tin- *!!'.<. futures have 

 annular lobes except in Pselioceras. Siphuncle ventncl f centra in tl //.,/< 

 becoming dorsad in later stages. 



PeeKoceras, Hyatt. Permian. Pleuronautilus, Mojs. ; A'//"'//oc/Trt, gen. nov. 

 Type E. (Pleur.) superbus, Mojs. sp. ; Enoploceras, gen. nov. Type E. (Naut.) ll'ulf' m\ 

 ]\Iojs. sp. ; Anoploceras, gen. nov. Type A. (Pleur.) ampezzanus, Mojs. sp. Ti-ia>. 



VII. KONINCKOCERATIDA. 



Xnatilicones with biangular sections at an early stage of iimn-tli, ifi-ri'lnjiing ktter 

 into modified trapezoidal outlines as in many of. the Hercoceratida, but shells are .s-///"o/A, 

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

 finally 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 

 during the gerontic stage. 



Family 21. Koninckoceratidae. 



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

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

 have marked umbilical saddles. Volutions with broad umbilical zones which become 

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



Koninckoceras, Domatoceras, Hyatt. Carboniferous. Potoceras, Hyatt. Devonian (1).. 

 Stenopoceras, Peripetoceras, Hyatt. Permian. 



Family 22. Solenocheilidae. 



Compressed elliptical in section during early stages, but full-grown of jiriiniffr,- /'.-///,.> 

 mill i/nung of specialised derivatives have a more or less trigonal section in //'/// stm/,-. 

 Later this stock evolves shells with volutions having depressed elliptical or brnm/lii li> mi- 

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

 umbilical shoulders. Umbilical zont ft ry l>r<i<xl, the increase by growth of flu </nr*iiin 

 being remarkably rapid. Shells smooth, <'mt <ij>erture in gerontic >/// mnii <l,,-,ln^ 

 peculiar lateral projections, especially at the umbilical shoulders, v:Jii<-h are //.</!///// very 

 prominent. Siphuncle sub-ventran. 



Aipoceras, Oncodoceras, Hyatt; Asymptoceras, Ryckholt ; Solenocheilus, Mt-i-k. 

 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. Carboniferoiis. 



