174 G. LINDSTRÖM, ON THE SILURIAN GASTEOPODA AND PTEROPODA OF GOTLAND. 



Gen. CYCLONEMA Hall. 



Gydonema 1852. Hall Pal. N. York vol. II, 89. 



Shell turhinate, lohorls ventricose, finely striated or carinated by longitudinal ridges, 

 wliich are niore developed than the transverse ones, which often are wanting. The shell has 

 been nacreous, as can be seen by the traces left hy the interiör stratum on the nucleus. 

 The operculwn is broadly conical, with some ten large coils outside, ivipressed by a shallow 

 groove along their superior börder and streaked by oblique, transversal lines. 



This genus comprises Litorina like shells, which differ from Oriostoma chiefly 

 through their operculum and through the regularly predominating, longitudinal orna- 

 mentation. In consequence of their characteristic operculum, which so much resem- 

 bles that of the Turbinida3, they cannot any longer be regarded as Litorinaä as Stoliczka 

 has proposed. 



In the strata of Gotland twelve species- have been found and in the Lower Silu- 

 rian of Dalecarlia and of Öland there have been detected some well preserved speci- 

 mens of this genus. 



1. Cyclonema delicatulum n. 



Pl. XV figs. 27—44. 



This shell, being one of the raost common and characteristic of the Silurian for- 

 mation of Gotland, is, as may be seen by the many figures and b}' the dimensions be- 

 low, one of the most variable, with forras ranging between elongate Murchisonia like 

 shells to depressed globular ones like Natica. But the many transitional forms and 

 above all the characteristic ornamentation unite them. They are, moreover, found in 

 the same stratum and often on the same locality. The most common variety is ligu- 

 red on plate XV fig. 28. It has four or five whorls, ventricose, with deep suture. 

 The body "whorl is almost as long as all the others taken together. The ornamenta- 

 tion of the surface is a most delicate net work of fine, sharply elevated spiral lines 

 and equal sized transverse lines, which intercross them and at the meeting point form 

 a small blunt tubercle. The surface of each quadrangle, enclosed by these lines, is 

 with the aid of a magnifying lens seen to be minutely and transversally lineated. The 

 figures 42 — 44 on pl. XV, shoAv how the form of these quadrangles varies in different 

 species according to the distance and position of the crossing lines, being elongate or 

 equal sided or transverse. Besides the now mentioned, prevalent type of the shell, there 

 are more elongated ones, of 6 or 7 whorls, the body whorl being equal to a third of 

 the whole length. The most extreme of the elongated ones is figured. In figures 

 40 — 41 the outlines of its antipodes are given, almost globular with enorraously large 

 body whorl and short spire. The specimen figured on pl. XV fig. 45 is probably only 

 a corroded specimen of this form from the shale of Wisby. 



The aperture is rounded, the exteriör lip sharp and thin, entire, the inner one 

 thin, reflexed. There is no umbilicus. The common variety has in length 12 mill.. 



