KONGL. SV. VET. AKADEMIENS HANDL. BANU. 19. N:0 6. 



61 



muscular scars as in Capulus, the globular, low or high spired fortus which almost 

 insensibly merge into each other through uuinerous gradations unite them all at 

 least the Silurian ones, in one genus, which althoiigh probably a near ally to Ca- 

 pulus, still must be considered as distinct. In accordance with Hall and the Ameri- 

 can authors I retain the genus Platyceras for the Silurian species. But I also think it 

 proper to unite within its limits the genera Platystoma and Strophostylus. These, as 

 may be seen by the following comparison, are indeed not much differentiated. Accor- 

 ding to the diagnosis given by Hall') they are thus characterized: 



Platyceras. 

 »Shell depressed, subglo- 

 bose, subovoid or obliquely 

 subconical. Spire small, vo- 

 lutions few, sometimes free 

 and sometimes contiguoi 

 witliout columella, aperture, 

 more or less expanded, often 

 campanulate and sometimes 

 with the lip reflexed: peri 

 stome entire or sinuous». 



spire 

 large 



Platystoma. 



»Shell subglobose; 

 short; aperture very 

 suborbicular, dilated; labrum 

 joining the body whorl at right 

 angles to the axis of the shell» 

 Conrad; »having columella, 

 columellar lip thickened.» 

 Hall. 



Strophostylus. 

 »Shell subglobose or ovoid- 

 globose. Spire small, with a 

 large ventricose body-whorl; 

 outer lip thin, not refiected; 

 columella twisted or spirally 

 groovecl within, not refiected; 

 uuibilicus none: aperture so- 

 mewhat round-ovate ortrans- 

 versely broad oval.» 



Consequently, the cliief differences are that in Platyceras there is no colu- 

 mella and in Strophostylus a twisted columella and Platystoma is nearly related to 

 Strophostylus. In the latest volurae of »Palfeontology of New- York», vol. V pt. II p. 

 129, Hall seems to hesitate about the distinction of the two first genera. »Platystoma, 

 he writes, »is in some species scarcely separable from Platyceras by any persistent 

 characters». Bakrois and other låter authors agree with Hall in this point. But Stro- 

 phostylus is also not tenable as the twisting in the columellar part of the reflexed 

 lip is seen in specimens which else agree with Platyceras. Pl. II fig. 37 — 38. Even 

 if Platystoma did not Coincide entirely, as it seems to me, with Platyceras, this name 

 could not be retained, as it has been preoccupied already several times before^). If 

 we now turn our attention to the numerous figures given by Hall and to specimens 

 from North America, we cannot fail to remark the extreme amount of variation, through 

 which the forms are mixed up with each other. It is indeed very difficult to find 

 any difference between the reflexed lip along the body whorl in Platyceras and Pla- 

 tystoma and the so called »twisted columella» in Strophostylus, which is in uninter- 

 rupted continuation with the outer lip and has no such separate callosity as in Natica 

 for instance. Moreover, in many of the specimens figured on plate XI, vol. V pt. II 

 of the Palaiontology of N. Y. the »columella» is neither twisted nor grooved. In con- 



1) Pal. of N.-Yoi-k, vol. III pp. 299, 303, 309. 

 -) 1753 Platystoma Klein (= Helix, Ampularia, Natica & Ncrita). 

 1803 Platystoma Meigen a dipterous insect. 

 1829 Platystoma Agassiz a fossil fish, 

 and moreover Laube in his work »Gastropoden der Hallstätter Scliioliten 1855, has also iiaraed a new genus 

 of his Platystoma. 



