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



near the centre. It is extremely thin, more so than in the precedinir two forms. The 

 coils are narrow, numerous and regular. Their sides are ornamented by numerous, thin 

 and narrow lines, nearl}' ])arallel with the coils, but sectioning them in slight obliquity. 

 Liongest diameter 12 millini., shortest diam. 10 mill. Thickness of margin scarcely 1 

 millira. 



If these three forms of opercula belong to a common generic type, as I think, 

 the following one differs so essentially from them, that it must be regarded as belong- 

 ino- to an other genus, which for the present is quite as unknown as the former. 



4. PI. XVII tig. 57. From the shale beds near Wisby four specimens of the lar- 

 gest operculum known in Gotland, have been obtained. But only the outside is visi- 

 ble, the extremely thin shell firmly adhering with the inside to the soft shale. It is 

 circular, fiat, covered by thick laminar, elevated, spirally wound coils, in all twelve, 

 rhey have broad bases and obtuse tops. They leave between them deep interspaces, 

 nearly as large as themselves and they lean with their tops towards the centre. The 

 largest diameter is 32 mill, and it must consequently have appertained to a very large 

 shell, possibly to Cyclonema? giganteum, of which a few specimens are found in the 

 same beds. 



The opercula of the Oriostoma pattern have sometimes been compared with 

 those extant in several recent genera as Torinia and Omalaxis. The operculum of 

 Torinia is, however, constructed upon a quite different plan, its inside is wholly dis- 

 similar, protruding in the centre in a rod like })rolongation, while the operculum of 

 the paUcozoic shells is sunken in the centre. Moreover, both Torinia and Omalaxis 

 have entirely chitinous opercula, whilst the palaeozoic ones evidently from the beginning 

 were shelly. I have not found any other opercula resembling the palaeozoic ones more 

 than that figured by D'Orbigny in his ')Paleontologie Frangaise, Terrains Cretaces», pi. 

 186 bis, f. 18 — 17 and which, according to him, 1. c. p. 228, has belonged to unknown 

 species of Turbo. It comes nearer to those of Cyclonema, than those of Oriostoma. 

 D'Orbigny remarks that the coils are wtres rapproches, comme chez les Trochus pro- 

 prements dits». And, in fact, the opercula of Cyclonema, of which see below, in a 

 nigh degree resemble the operculum of the Trochoid Livonia as to the outside, espe- 

 cially in the ornamentation by fine, oblique lines. 



11. Oriostoma lielicinum n. 



PI. Ill tig. 27— .31, PI. XX fig. .30—33. 



Shell globular, heliciform, with short spire ending in a blunt point, whorls four 

 and a half, convex; suture shallow. Ornamentation consisting only of a succession of 

 fine, transverse lines bending backwards on the median line of the last whorl and then 

 again forwards on the umbilical side, very close and with smooth intei'stices. They 

 are arranged as it M'ere in small groups divided through shallow furrows which give 

 the shell a wavy contour. There are absolutely no traces of any longitudinal lines even 

 in the best preserved specimens and the surface of the whorls is glossy and shining 



