346 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
frequently found floating on the surface masses of vegetation, more or 
less water-logged, and ready to sink. The contents of some of our 
trawls would certainly have puzzled a paleontologist; between the 
deep-water forms of crustacea, annelids, fishes, echinoderms, sponges, 
etc., and the mango and orange leaves mingled with the branches of 
bamboo, nutmegs, and land shells, both animal and vegetable forms. 
being in great profusion, he would have found it difficult to decide 
whether he had to deal with a marine or a land fauna.” ! 
It is not improbable that similar conditions obtained in Silurian 
days. Eurypterids occur at about the same level, and their presence 
is susceptible of the same explanation, even if they were not them- 
selves marine. 
In the south of Gotland I do not consider that any evidence of an 
unconformity indicating a land interval is present nor was any seen in 
the waterfall section at Wisby, but the observations made at that 
section were not extensive, nor detailed. Hedstrém described a dis- 
conformity between the Lower and Upper Gotlandian in the precipice: 
south of Gustafsvik, indicated by a “thin marly and gravelly water- 
worn layer, containing, amongst other things, worn Gastropods of the 
genera Trochus, Pleurotomaria, Horistomia etc.,” and he has little 
doubt of the actual occurrence of erosion, but he does not state whether 
he considers this as indicative of a land interval.? I have not seen the 
locality and so can not give further data, but the presence of pebbles 
and worn shells may be equally well explained by contemporaneous. 
erosion in a coral reef channel. It appears that the question of land 
in Middle Silurian time, within the present area of Gotland, remains: 
to be proven, and the idea is favored that marine conditions prevailed 
over the present limits of Gotland throughout the time of deposition. 
embraced between the youngest and oldest deposits. 
Evidence of shallow water. That the Gotland rocks are the deposits. 
of very shallow water is evidenced by the extraordinary development 
of coral reefs, the strongly developed edaphic modification shown by 
the fossil faunas, and the extensive lateral gradation of sediments. 
These show that the sea bottom was sufficiently shallow to closely 
respond to land conditions, and that there were numerous local bar- 
riers so near the surface as to produce a great variety of differing con- 
ditions. Among these local barriers the most important were the 
coral reefs which protected colonies within them and hindered migra~ 
tion over, or around them. 
1 Agassiz. Three cruises of the Blake, 1888, 1, p. 291. 
2 Hedstr6ém. Guide book 11th. internat. geol. congr., 1910, no. 20, p. 23, fig. 4. 
