KONOL. SV. VKT. AKADKMIEN.S HANDLINGAH. HAND. 19. N:U 6. 109 



The Gotland specimens are only nuclei and consequently the details of the sur- 

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



In tlie oldest shale beds near Wisby six specimens or nuclei have been found in 

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

 througli the dimensions given, they are more Hal than those from tlie Lower Silurian 

 strata. Largest diameter 4;> niillim., height 1.5 millim. Breadth of the largest wIkjiI Hi 

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

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

 cimen from Borkholm in Esthonia has the aperture of 18 mill, in height and 20 mill, 

 in breadth. Specimens which nearest resemble this are first met with in the upper- 

 most strata of the Lower lied Orthoceratite limestone from Killla in Gland. It then 

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

 Dalecarlia and Gland 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 uj) 

 the distinctive features of these very common fossils. 



1. Pleurot. qualteriata SciiLOTU. 1820, with Hat, horizontal or faintly convex apex, 

 slit baud 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 Suiilotheim, when he com- 

 pares it witli Helix gualteriana, had intended to name it gualteriana or gualteriata, 

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

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



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

 with prominent apex of steplike whorls, slit band (pi. XIII iig. 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 calcai-eous matter. 

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

 carlia. Then there are two other species which belong to a (juite difterent type, as 

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



1. Pleurot. mdrt/inata 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 wide, 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 L^pper Gray Orthoceratite limestone of Gland. A variety, 

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

 youngest limestone beds of Gland. There are several diaphragms in the apex of this 

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

 Salteu in oCanad. Organ. Remains'). Dec. 1 p. 16, through the slit band is more de- 

 veloped in PI. marginata. 



