174 G. LINDSTROM, ON THE SILURIAN GASTROPODA AND PTEROPODA OF GOTLAND. 



Gen. CYCLONEMA Hall. 



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



Shell turbinate, whorls ventricose, finely striated or carinated by longitudinal ridges, 

 which are more developed than the transverse ones, ivhich often are u-anting. The shell has 

 been nacreous, as can be seen by the traces left by the interior stratum oi\ the nucleus. 

 The operculum is broadly conical, with some ten large coils outside, impressed by a shallow 

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



This genus comprises Litorina lil^e 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 Turbinidte, they cannot any longer be regarded as Litorina:; as Stoliczka 

 has proposed. 



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

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

 mens of this genus. 



L Gyclonema delicatulum n. 



PI. XV figs. 27—44. 



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

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

 low, one of the most variable, with forms 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 figu- 

 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 pi. XV, show how the form of these quadrangles varies in diff^erent 

 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 enormously large 

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

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



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

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



