15 2 G. LINDSTRÖM, ON THE SILURIAN GASTKOPODA AND PTEROPODA 01'' GOTLAND. 



encircling its middle line. 'J'his keel is situated just below the suture in the older 

 whorls, which are hidden as to their upper part. The surface is striated by oblique, 

 exquisitely fine, a little serriilated lines, directed backwards. The deeply incised suture 

 is most characteristic though there are variations as to its depth. For instance a shell 

 from Follingbo, fig. 19, same locality as fig. 26 — 27, has rather no incision at the 

 suture or one only very shallow, the inferior börder of each whorl beginning further 

 down on the next preceding. The aperture is circular, the outer lip thin, the inner 

 lip refiexed. The umbilicus is narrow, but open. H. 17 mill., br. 17 mill. Length 

 of aperture 9 mill., width 10 mill. Several specimens have been found at Kyrkberget 

 in Wisby, Samsugn in Othem, Stor Wede in Follingbo; from Likkcrshamn there are 

 specimens in the Mineralogical Museum of Copenhagen. 



14. Trochus gyrans n. 



Pl. XVIII, fig. 18—20. 



Shell regularly conical of eight tumid whorls, which are carinated along their 

 median line and just above the suture form a horlzontal, flat surface like a narrow 

 band along the inferior part of the whorls, bordered upwards by a narrow ridge. 

 The sculpture consists in fine strios, which are somewhat more irregular than in the 

 preceding. The aperture is circular and the outer lip thin, the inner one thickened 

 and there is the appearance of a sinus where both lips meet at the lower end of the 

 aperture. The structure of the umbilicus is peculiar. It is a wide, funnelshaped 

 opening, limited partially by the inner lip and partially by a sharply defined wall, 

 which is encircled by a narrow ridge and is continued spirally down to the more 

 narrow and deeper opening. H. 19 millim., br. 15 mill. 



Two specimens have been found on Kyrkberget in Wisby together with the pre- 

 ceding, to which it is nearly related. It differs, however, chiefly thro\igh its peculiar 

 and characteristic umbilicus. 



15. Trochus densestriatus n. 



Pl. XVIII fig. 21—23. 



Shell minute, elongately conical of six whorls with slightly concave outline. 

 Beneath the shallow suture there is a low, elevated ring around the upper edge of the 

 whorls, which on the body whorl is situated exactly where the apical and the umbilical 

 surfaces meet. There are regular, dense, microscopically fine streaks, riinning nearly 

 vertical on the whorls, slightly arched. The aperture is rather obovate and the lips 

 thin and sharp, the outer one bent inwards and the inner one narrowly reflexed, no 

 umbilicus is visible. H. 5 millim., br. 3 mill. Four specimens have been collected in 

 an earthy, bituminous limestone at Hessle in Östergarn. 



It comes near to Tr. incisus, but is sufficiently woll distinguished through its 

 peculiar striation and the angular aperture. 



