120 G. LINDSTROM, ON TIIK SILURIAN GASTROPODA AND PTEROl'ODA OF GOTLAND. 



other whorls. It shows in the margin, fig. 12, the ends of the enclosed crescents, 

 which are formed upon the same plan as in the other wAlata^n. In a transversal sec- 

 tion, fig. 13, the uncommonly thick shell substance of the bordering lines encloses a 

 most narrow slit band. Beneath the slit band an angular ridge is seen and below that 

 the wall slopes abruptedly inwards to the suture. The surface is transversally divided 

 into large sinuous folds, the interstices being streaked with minute, parallel lines, fig. 

 12. The umbilical side, tig. 10, is comparatively smooth and glossy, the transverse lines 

 being most conspicuous near the margin, where, a little inwards, a blunt rib runs pa- 

 rallel with it, corresponding to a similar one on the lower side, though not so distinct. 

 Another ridge, more distinct, encircles the umbilicus, which is open, but narrow. The 

 aperture is circular as well as the section of the whorls, the lips are thin and sharj)- 

 edged without being reflexed. 



Dimensions of the specimen figured: height 12 millim., breadth 20 mill., aperture 

 7 mill. 



Several species are found in the shale beds of Wisby and Petes vik in Habblingbo. 



83. Pleurotomaria frenata n. 



PI. XI fig. 14. 



Shell turbinate of five ventricose whorls. Slit band situated only a little above 

 the median line of the whorls, its keel is short, of triangular section, with broad base 

 of plaited folds, fig. 14, with angular ends. The crescents of the band are as wide 

 apart at their ends as in the other species. The surface on both sides is covered by 

 fine, minute stria", wavy and interlacing, forming a reticMilate surface like that of the next 

 species. The strife are directed towards the aperture and turn backwards oiil}- on 

 th(! slit band. The aperture is regularly circular with thin, sharp lips, tlie uudiilicus 

 is o])en. 



Dimensions. Height 19 millim., breadth 26 mill., aperture 11 millim. Five spe- 

 cimens have been found in the shale beds near \Visb3\ 



This species is intermediate between Pleurot. pra>texta and Pleur. undulans, uni- 

 ting a slit band like tliat of the former with an ornamentation resembling that of the 

 latter. 



84. Pleurotomaria undulans n. 



PI. XI. Ilti-. 15—2.'!. 

 Eiinmpli. (rquilntcrus 1807. LiNnsTK. Nomina, 23. 



Shell helicoid, thin and fragile, and consequently usually cru.shed and distorted, spire 

 short, whorls si.x, ventricose. Slit band situated nearly on the middle line of the body 

 whorl, on the other whorls close to the upper suture, which it covers, very narrow and 

 often enlargencd into a thin lamina, which is bent and folded into a succession of irregular 

 waves, as delineated in the figures. Its interior structure coincides entirel)' with that 



