172 G. LINDSTRÖM, ON THE SILURIAN GASTROPODA AND PTEROPODA OF GOTLAND. 



Shell discoidal, planorbiform, spire of eight whorls, depressed or only sliijhtly 

 prominent. Whorls cylindrical, terete with several sharp keels, which are more pro- 

 minent and distinct in young specimens than in older, where they are more or less 

 obliterated. On the apical side there are two sharp and prominent, narrow keels, lea- 

 ving a flat band between them, one close to the suture, the other and larger on the 

 outside. Between the interiör keel and the next whorl a deep, narrow groove is going 

 and the innermost apical whorls are in many instances sunk as a shallow depression. 

 The middle of the body whorl and also of the others is encircled by a highly acute 

 keel and on the umbilical side there are at least three such, forming between them 

 sharply marked facets, of which the lowest is abruptly turned down. On the umbi- 

 lical side there are thus at least four different faces. The apical keels disappear du- 

 ring the continued growth of the shell and instead of the seven original keels there 

 are on the body whorl finally only four or five on the umbilical side. The contour 

 of the whorls is, when all keels are present, a regular octogon, fig. 37, as the surface 

 between the keels is nearly straight or only slightly concave. The fine, transverse strise 

 run almost straight down över the keels, only a little bent forwards on the keels. In 

 fullgrown or large specimens the aperture is circular and the umbilicus is wide and 

 open, more so than in most of the other shells. There is some variability, for instance 

 in several specimens from Follingbo, where the whorls on the apical side are so much 

 sunk, that there is a spire on the umbilical side. In some there are no keels at all 

 on the larger whorls, not even near the umbilicus, and the whorls are uniformly tu- 

 bular. There is also a tendency in the body whorl to disjoint itself from the other 

 whorls, as remarkable in specimens from Fårösund. Br. 66 mill., h. 18 mill. 



The figure given by Hisinger of his Inachus angulatus is bad and exaggerated. 

 There ai"e four specimens with Hisinger's hand named thus in his own collection, of 

 quite the same variety as mine. But the type specimen of his figure is not ainongst 

 them, it is probably from the Museum of Upsala. That Wahleni5erg's Helicites supra- 

 angulatus is identical with this species, is corroborated through his type specimen, 

 which I have been able to see through the kindness of Prof. Cleve and it quite agrees 

 with the large specimens from the limestone of Wialmsudd. 



Salter^) says that »Inachus angulatus Hisinger, is probably an extreme form of 

 the group (of Trochonema) with a greatly depressed spire». This statement can hardly 

 be accepted and the fossil in question is here retained within Oriostoma. It may 

 through future researches be decided whether it should not rather be placed with the 

 Euomphali, from which it, however, differs in the want of any notch in the last whorls 

 and the concomitant ridges. The transverse strise are, moreover, hent fo7'ioards on the 

 keels, not backwards. 



This is a very abundant and well preserved shell in some localities: Länsa and 

 Lutterhorn of Fårö, Wialmsudd and Sandviken at Fårösund, Länna near Slite, Sams- 

 ugn in Othem, Qvarnbacken of Slite, Kylley, Enholmen, Barabacke, Follingbo, Kålens 

 Qvarn, Wisby in the uppermost limestone, Klinteberg, Stora Carlsö near Altaret. It 

 thus occurs exclusively in the uppermost, crystalline limestone. 



') GeoL Survey of Canada, Cauad. Org. Remains, Deeade I, 27. 



