KONGL. SV. VET. AKADKMIKNS HANOLINGAli. BAND 19. N:0 6. 63 



however, not bordered by elevated lines as in the Pleurotomarida?, they resemble more 

 what is seen in the recent Ianthina3, which also sometimes exhibit such notches. 



Probably much in consequence of this slight resemblance with the last inentioned 

 group, some authors have given Platyceras (— - Platystoma) its systematic place with 

 the lanthinas. Hall in the fifth vol., lid part of Pal. of New- York, has signed the 

 plates on the Plat5'cerata as lanthinidaä for Platystoma and Strophostylus, and Platy- 

 ceridas for Platyceras, but in the descriptive letterpress he is willing to unite all three 

 genera into one genus. The thin and fragile shell of lanthina is not corresponded by 

 the thick and heavy one in Platyceras. Other authors, again, as Stoliczka') and Waa- 

 GEN^) see the affinity of the Platycerata with Velutina, to which several of them show 

 some resemblance in the exteriör shape of the shell. Platyceras may, however, as has 

 been shown above, most fitly be ranked with the Capulida^, with which they also share 

 the peculiarity in habit of fixing themselves closely with the aperture to the surface 

 of other marine animals, especially Crinoids. 



In Sweden the oldest Platycerata found are from the beds of the Leptsena lime- 

 stone of Dalecarlia, from where five different species have been described ^). At least 

 two of the Gotlandic species are also found in the Upper Silurian of Scania. The 

 shclls of this genus have been the most common of all Gastropoda in the pateozoic 

 seas. While in the Silurian epoch Platyceras cornutum and others of spirally coiled 

 forms were prevalent, in the Devonian of Harz and North America and still more so 

 in the Carboniferous strata, especially the North American, the straight forms which 

 constitute the subgenus Orthonychia are most abundant. 



The Gotland species may be arranged as follows: 



I. Platyceras proper, with spire coiled of three or four whorls and almost en- 

 tirely or nearly contiguous with the body whorl. 



1. Pl. cornutum His. 



2. Pl. cornutum, var. loricatum n. 



3. Pl. prototypum Phillips. *) 



4. Pl. disciforme n. 



II. Orthonychia spire diminutive or evanescent, shell straight, tubular. 



5. Pl. enorme n. 



6. Pl. cyathinum n. 



1. Platyceras cornutum IIisinger. 



Pl. II (igs. 29—51, pl. III figs. 6—9, 19—26. 



TurMnites. 1828. HisiNGEU Ariteckn. IV, 221, 1,ab. VI f. 6. 



Pileopsis cornuta. 18.37. Hisinger LeUirea Sueciea, 41, pl. XII f. 11. 



1848. Bronn Noraenclator, 97.3. 



Pileopsis sulcata. 1837. Hisingeu Lellinea, 41, pl. XII f. 12. 



1848. Bronn Noraenclator, 974. 



') Cretacoous Ifos.sils of India p. 319. 



-) Salt-Raiige Fossils p. 103. 



•'') Angeltn & Lindström. Prag-m. Sil. p. 14. 



*) By inadvertence this species was naraed P. spirabnn Sow. in the list, page 16. 



