KUNGL. SV. VET. AlvADEMIENS llANDL. BAND. 19. N:u 6. 



61 



uiiisciilar scars as iu Capulus, the globulai-, low or higli spired forms which ahuost 

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

 least the Silurian ones, in one genus, which although 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 1 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') tliey are thus characterized: 



Platyceras. 

 ))Shell depressed, subglo- 

 bose, subovoid or obliquqly 

 subconical. Spire small, vo- 

 lutions few, sometimes free 

 and sometimes contiguous 

 without columella, aperture, 

 more or less expanded, often 

 campaiiulate and sometimes 

 with the lip reflexed : peri 

 .stome entire or sinuous». 



Platystoma,. 

 »Shell subglobose; spire 

 short; aperture very large 

 suborbicular, dilated; labrum 

 joining the body whorl at right 

 angles to the axis of the slielL) 

 Conkad; "having columella, 

 columellar lip thickened. » 

 Hall. 



Stroi)hostylus. 

 wShell subglobose or ovoid- 

 globose. Spire small, with a 

 large ventricose body-Avhorl; 

 outer lip thin, not reflected; 

 columella twisted or spindly 

 grooved witldn, not reflected; 

 umbilicus none: aperture so- 

 mewhat round-ovate or trans- 

 versely broad oval.)) 



Consecjuently, the chief differences are that in Platyceras there is no colu- 

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

 Stro])hostylus. In the latest volume of »Pala3ontology 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 later 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. PI. 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 Palaeontology of N. Y. the »coluraeIIa» is neither twisted nor grooved. In con- 



>) Pal. of N.-Yoik, vol. Ill pp. 2"JU, 303, 309. 

 ^) 1753 Platystoma Kletx (= Helix, .\rapularia, Natica & Ncrita). 

 1803 Platystoma Meigen a dipterous insect. 

 1829 Platystoma Agassiz a fossil tish, 

 and moreover Laube iu his work "Gastropoden der Hallstiittcr Schichten 1855, lias also named a new genus 

 of his Platystoma. 



