96 G. LINDSTTIÖM, ON TIIE SILURIAN GASTROPODA AND PTEROPODA OF GOTLAND. 



2. Pleurotomaria gradata n. 



Pl. VII figs. 26—27. 



Shell conical, trochiforra, spire blunt, broad, whorls eight with regularly straight, 

 sloping sides of faint coiivexitj', abruptedly bent aloiig the superior siiture through 

 the vertical and almost even zone around the slit band. This band is central on the body 

 whorl and remarkable as being one of the narrowest amongst all the Silurian Pleuro- 

 tomaria; and perhaps amongst all known. It is reduced to a narrow fissure, a mere frac- 

 tion of a millimetre and thin as a knife edge. It continues open a long way round 

 the body whorl and the next, and then at last is filled np. Biit owing to its narrow- 

 ness it is impossible to discern any crescents on it. Like the related, preceding one, 

 the slit band is enclosed within a separate zone, differently ornamented from the other 

 surface and bordered by a narrow line on each side. In this median field there are 

 only transverse lines crossed by a few, obscurc, longitudinal ones. On some distance 

 a blunt ridge is running on each side. The ornamentation of the whorls consists of 

 somewhat distantiated, transverse lines, connected by raore narrow spiral ones and 

 parallel to the slit band; in the interstices microscopically minute transverse lines are seen. 

 The belt around the slit band does not form any prominent ridge as in the former 

 species, but rather a somewhat concave or straight facet on the whorl. The suture is 

 shallow. The aperture is rather more broad than high, its columellar lip is strongly 

 reflexed around the open umbilicus. 



Height 22 millim., breadth 20 millim. Apical angle 69°. Anöther specimen has 

 24 millim. in height and 23 mill. in breadth. Found in numerous specimens in the 

 limestone of Sandarfve kulle and also in the gray limestone of Linde kulle. 



Oehleut has described a Pl. Virensis from the Lower Devonian of France, and 

 this comes near to our species through its ornamentation and the slit band, though 

 its spire is lower. 



3. Pleurotomaria Linnarssoni n. 



Pl. XX fig. 21—24. 



Shell small, discoid, spire moderately prominent, whorls six, incrcasing in size 

 slowly and proportionatcly. They are adorned with a delicate network, on the apical 

 side consisting of intercrossing strite of which the transverse ones are directed back- 

 wards in an aciite angle ao;ainst the slit band. At the intercrossino' of these lines 

 small nodules are formed giving the surface a prickly appearance, with small pits in 

 the interstices. The slit band is placed below a narrow keel which divides the shell 

 exactly in two halves. It is exceedingly narrow and accompanied on both sides by 

 longitudinal lines, which are more prominent than the others. On the umbilical side 

 the whorls are more ventricose than on the opposite and also similarly ornamented. 

 The aperture is circular and the umbilicus is large and open. H. 4 mill., br. 8 mill. 



