BIIIANG TILL K. SV. VET. .\KAU. HANDL. B. 3. N:0 12. 17 



posterior side flattened, thus forming a sort of false area. This 

 area is often somewhat depressed longitudinally, but without any 

 distinct groove. Beak pointed, not very excentric; profile from 

 the beak to the anterior margin straight, from the beak to the 

 posterior margin usually very slightly convex. The siirface is 

 smooth, as far as I can see. At the apex I have not been able 

 to discern any foramen, but I have only seen a very few spe- 

 cimens Avith the shell preserved, and, owing to its extreme mi- 

 nuteness, the foramen, even if extant, might be very difficult to 

 discern. The interiör is not visible in any of the specimens at 

 hand, but the internal cast (tig. 34) shows at the apex a groove, 

 bordered by two longitudinal, subparallel ridges. — The dorsal 

 valve is very slightly and uniformly convex, marked with con- 

 centric lines of growth. Its beak is marginal. The interiör is 

 unknown. — The diameter of the shell seldom exceeds 2 mm., 

 the Icngth being usually very little less than the width. The 

 height is about half the width. 



Acrotreta socialis was originally described by Professor V. 

 Seebach from the regio Conocorypharum, or the strata with 

 Paradoxides Forchhammeri, in the island of Bornholm. Though 

 the species I have befbre me does not agree in every respect 

 with the above cited description and figures by Professor v. See- 

 bach, 1 do not hesitate to adopt for it the specific name given 

 by him. It occurs abundantly in the strata with Paradoxides Forch- 

 hammeri in the island of Bornholm, whence it has been sent me 

 by Professor Johnstrup under the name of A. socialis. The 

 want of agreement in certain instances between my specimens 

 and the description of Professor V. Seebach is probably due to 

 the circumstance that he has, as it seems, made up his descrip- 

 tion from specimens belonging to more than one species. That 

 he has included in his Acrotreta socialis really different species 

 may be inferred with great probability even from his statement 

 that it occurs also in the Graptolite schists. Hence he concludes 

 that these also belong to the regio Conocorypharum, but it is 

 quite unquestionable that the Graptolite schists occupy a much 

 higher horizon, ^) and it is not probable that they have any spe- 

 cies in common with the regio Conocorypharum; at least I have 

 not found, or seen in the coUections from Bornholm any species 



') The Graptolite schists referrecl to by Professor v. Seebach are, no doubt, 

 the middle Graptolite schists (or Dicranograptus schists), the place of 

 which is about the middle of the Lower Silurian. 



2 



