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of which the best known form is Discitoceras (Naut.) discors, sp. 

 McCoy. All species have large umbilical perforations, the nepi- 

 onic stage has no dorsal furrow and there is only a contact furrow 

 generated in later stages. 



Koninckioceratid.ee. 



The genera Koninckioceras and Domatoceras have been exam- 

 ined and these have no impressed zone until after contact. 



Solenc ch e Hi dee. 



In Aipoceras, the arcuate form of this family, there is no impressed 

 zone, and in Oncodoceras, some of which last are gyroceran, no 

 impressed zone has been observed. In Asymptoceras, although 

 the forms are all close coiled, this zone is only faintly indicated in 

 some species, and when it is better defined it occurs late in the 

 ontogeny, and then only as a contact furrow. The same remarks 

 apply also to species of Solenocheilus. In the description of Cran- 

 oceras this genus was referred to as comparable with Cranoceras 

 depression in the peculiar configuration of the dorsum. 



There are however, differences which show that this resemblance 

 is not very close or significant. In Solenocheilus Springer/, for ex- 

 ample, the gibbous dorsum and latero-dorsal flutes and heavy ridges 

 on the umbilical shoulders appear before the contact furrow, but 

 this comes into existence as soon as the whorls touch and modifies 

 the dorsum in proportion to the amount of involution. The latero- 

 dorsal flutes also are dependant upon the extent and size of the 

 ridges or keels on umbilical shoulders and are not as in Cranoceras 

 primitive inflections of the dorsal surface. 



Incerta Sedes. 



The new genera Peripetoceras of the (Permian) Dyas and Syrin- 

 goceras of the Trias together with Mojsisovics genus Pleuronautilus 

 of the Trias have been given together here as a matter of conve- 

 nience, although it is by no means certain that they belong to the 

 same family. 



Peripetoceras,* n. g. 



This genus has been instituted for a single species described 

 below, which cannot be placed with any other species. 

 IIspi-STTJs, clasped around. 



