KONGL. SV. VET. AKADEMIENS HANDLINGAR. BAND. 19. N:0 6. 109 



The Gotland spccimens iire only nuclei and consequently the details uf the sur- 

 face have been taken from Lower Silurian specimens of nearly the same form. 



In the oldest sliale beds near Wisby six speeimens or nuelci have been found in 

 sitn and thus there is certainty that they have not been transported. As may be seen 

 throiigh the dimensions given, they are more flat than those frum the Lower Silurian 

 strata. Largest diameter 43 millim., height 15 millim. Breadth of the largest whorl 16 

 millim., height of the same 11 mill., while in one of the Lower Silurian ones the 

 largest whorl near the aperture is 18 millim. in breadth and in height 13 mill. A spe- 

 eimen from Borkholm in Esthonia has the aperture of 18 mill. in height and 20 mill. 

 in breadth. Specimens whieh nearest resemble this are tirst met with in the upper- 

 most strata of the Lower Red Orthoceratite limestone from Källa in Öland. It then 

 occurs in numerous specimens in the Lower and Upper Gray Orthoceratite limestone of 

 Dalecarlia and Oland and also of other provinces, and returns again in the uppermost 

 limestone strata of the Lower Silurian. Coeval with this species there lived at least 

 one nearly allied species and two others which have been rather confounded with them. 

 The characters of all four are shortly given below, in order in some way to clear up 

 the distinctive features of these very common fossils. 



1. Pleurot. qualteriata Schloth. 1820, with flat, horizontal or faintly convex apex, 

 slit band largest on the apical side, only discernible through the direction of the sculp- 

 tural lines. 



As to the quaint name of this species it seems that Schlotiieim, when he com- 

 pares it with Helix gualteriaua, had intended to name it gualteriana or gualtcriata, 

 but that through soine error the g had been changed into a q. He, nevertheless, re- 

 tained the låter, without giving any clue to its derivation. 



2. Pleurot. obvallata Wahlenberg 1818 (Petref. Svec. p. 73 tab. IV fig. 1 — 2) 

 with prominent apex of steplike whorls, slit band (pl. XIII hg. 18) large on the api- 

 cal side, narrow on the umbilical side, in its centre with densely packed crescents, 

 nearly resembling those of Pleur. bicincta. This species is more common than the 

 former and it has, in the same way, filled up the apex with solid calcareous matter. 

 It is found in the Lower and Upper Gray Orthoceratite Limestone of Öland and Dale- 

 carlia. Then there are two other species which belong to a quite different type, as 

 they have the slit band built upon the same plan as prevails in the Division of the Alataj. 



1. Pleurot. marginata Eichwald. This forms the transition to the next ex- 

 treme species. It is large, with six ventricose, contiguous whorls, on the apical 

 side sunken in a Avide, open funnel, their inner face abruptedly sloping inwards. The 

 slit band on the inferior edge of the whorls is thin, lamellar, and winglike. The sur- 

 face is finely, transversally striated, on the umbilical side there are broad plaits. It 

 occurs in the Lower and Upper Gray Orthoceratite limestone of Öland. A variety, 

 with the youngest whorls uncoiled, belongs also to this species and occurs in the 

 youngest limestone beds of Öland. There are several diaphragins in the apex of this 

 shell. This species is related to the Canadian Ophileta as it has been described by 

 Salter in »Canad. Organ. Remains». Dec. 1 p. 16, through the slit band is more de- 

 veloped in Fl. marginata. 



