545 
of which the best known form is Désezfoceras (Vaut.) 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. 
Koninckioceratide. 
The genera Koninckioceras and Domatoceras have been exam- 
ined and these have no impressed zone until after contact. 
Solencchetlide. 
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 asa 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 
depressum in the peculiar configuration of the dorsum. 
There are however, differences which show that this resemblance 
is not very close or significant. In Solenochetlus Springert, 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. 
Ilepizet7s, clasped around. 
PROC. AMER. PHILOS. SOC. XXXII. 143, 3Q. PRINTED JULY 19, 1894. 
