KONGL. SV. VET. AKADKMIENS HANDLINGAR. BAND 19. N:0 6. 13 



this genus, as well as in Cyrtolites, Bellerophon and Oriostoina some speciraens still 

 show the pattern of the original colour bands. Traces of the nacreous lustre are also 

 present on the inside of several species of the genera Oriostoraa and Pleurotomaria. 

 A nurnber of specimens of the genus Oriostoina bas been found with their large, 

 calcareous operculum in situ, thus permitting to distinguish several, nearly allied 

 species, the shells of which show very little difference. With regard to the position of 

 the shells in the strata it may only be remarked that such elongated shells, as Mur- 

 chisonia, Loxonema and Subulites lie horizontally or parallel with the plan of strati- 

 fication. As for the rest they are heaped together without any order as the shells at 

 the present day cast ashore. 



The Murchisoniffi found in the shale beds of Wisby, and also many Bellerophons, 

 had been almost entirely overgrown by colonies of the bryozoan Ceramopora and others, 

 inany times larger than the shells themselves and thus evidently preventing all possi- 

 bility for the animal to move and consequently killing it. In a small lake south of 

 Stockholm, Haramarbysjö, I found in the spring 1882 the common freshwater bryozoan 

 Alcyonella fungosa Pallas in great abundance and nearly every colony of it had 

 chosen a specimen of Paludina vivipara L. for its basis and so completely covered 

 it that the animal was unable to move, and large numbers of them were dead, the 

 bryozoans continuing to thrive. Thus a powerful growth of a compound species has 

 acted and still acts as a check to the increase of another species belonging to a diffe- 

 rent group and has probably also been an agent in its extinction on localities common 

 to them both. 



Distribution of the Species. 



There is no doubt that the Gastropoda I am about to describe, numerous though 

 they be, are only a fragment of all that once made up the fauna of these animals in 

 the Silurian seas of Gotland. Several strata litterally teem with their broken shells 

 and it is evident by several of these remains that they belonged to many other spe- 

 cies than those mentioned further on, but too imperfect for description. As far as 

 known at present the number of species and varieties arnounts to 174 here described 

 and figured. The nuraeric strength of their group in relation to that of other orders 

 from the strata of Gotland may be seen from the following survej' of the whole Letha^a. 



List slaowing the numter of species at present known from the 

 Silurian Strata of Gotland. 



Criistaeea TrilobiUe _ 53 



Merostomata 2 



Phyllopoda 4 



Ostracoda .-. - _ 30 



Cirrhipedia_ 1 nn 



Annelida according to tlie researches of Dr J. G. 

 HiNDE with addition of the Tabicolar An- 

 nelides -.. 50 



Carried forward 140 



