KONGL. SV. VET. AKADEMIENS HANDLINGAR. BAND. 19. N:0 6. 157 



In certain species of Oriostoma, which have a single longitudinal keel near the 

 suture, there is at first sight some resemblance with species of Euoinphalus, but the 

 apertural sinus is here reduced to a short notch, which through the folding of the 

 aperture originates the keels. These notches never attain to such large dimensions as 

 the sinus in the Euomphalida3 and moreover they disappear in the body whorl of many. 



The shells of this genus are amongst the most common in the Silurian strata of 

 Gotland and through their large numbers they offer a great variety of forms, which 

 make it a most difficult task to distinguish specific groups neatly from each other. 

 The great variability of the ornamentation and the almost imperceptible passages from 

 one form to another highly increase this difficulty. The oldest notions of the Swedish 

 Oriostomata are those given by Beomell in his Lithographia p. 32, 36, where several 

 are described under the name of Cornu Ammonis, but only one clearly enough to be 

 identified, viz. J\S 21, »Ejusdem generis Nerititte majores» which is Oriostoma sculptum. 



In the Lower Silurian formation of Sweden hitherto not a single species has 

 been found belonging to this genus. In Gotland the following sixteen species and va- 

 rieties have been detected and they may be divided in two groups. 



A) with spire moderately prominent and the umbilicus deep and open. 



1. Oriostoma discors Sow. 



2. » var. rugosum Sow. 



3. O. contrarium n. 



4. O. globosum SCHLOTH. 



5. O. « var. sculptum Sow. 



6. O. coronatum n. 



7. O. acutum n. 



8. O. Wisbyense n. 



9. O. angulifer n. 



10. O. Roemeri n. 



11. O. helicinum n. 



12. O. alatum n. 



B) with short spire and planorbiform shell, the whorls on the umbilical side 

 quite as much visible as on the apical one. 



13. Oriostoma angulatum Wahlenberg. 



14. O. lineatum n. 



15. O. nitidissimum n. 



16. O. dispar n. 



Besides these species there is evidence of at least three species more through their 

 opercula, described below, but as no shell has been found to match them, they must 

 for the present be left undeterminated. 



1. Oriostoma discors Sow. 



Pl. XVI fig. 20—36, pl. XVII fig. 1—5. 

 Euomphalus discors 1814. Sow. Min. Conch. vol. I, 113, pl. 52 f. 1. 



1837. Agassiz in Sowebby Gross Brit. Mineral Conch. 61, tab. 36. 



