300 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
Etelhem Dr. Henry Munthe collected this species ! on the summit of 
one of the knolls of the Ascoceras limestone which are so abundant in 
that region, while on the summit of another knoll of the same region 
I collected Conchidium conchidium (Dalman), C. biloculare (Linné) 
in the midst of a coralline limestone of the Ascoceras type, and on 
Storo Karlso Pentamerus estonus Lindstrém and Conchidiwm conchi- 
dium occur in association in the midst of coralline limestones belonging 
to the rocks of the highest part of the island. Orthis rustica Sowerby 
is another species which can be found far above its natural strati- 
graphic position in the midst of the highest rocks of the island, the 
writer having collected it in clay pockets in the coral reef at Hoburgen. 
It has been quite generally considered that the highest rocks of 
Gotland are the youngest, yet we find therein species whose natural 
stratigraphic positions are lower in the section. | I believe that the 
high coralline rocks of Gotland are not the youngest of that island, 
that they are older than any rock (there are a few exceptions) which 
lies at the same level on their flanks, and that the coralline rocks 
should be correlated with strata holding a much lower level. A recent 
letter from Dr. Munthe shows that he is coming to the same conclusion 
for he says, writing under date of October 2, 1915, “Last summer I 
have ascertained that the ‘Marl Shales’ (Lindstrém’s bed e) are 
synchronous with a part of the reef-limestone (“Ascoceras limestone 
in part’) in Garde etc.” I believe the conditions are something like 
those represented in Figure 1. 
Conditions like these make local correlation extremely difficult, 
and when one considers that collections have been made without a 
recognition of the possibility of their occurrence, it is readily seen to 
what extent extra-Gotland correlation would be complicated. It is 
believed that in this way are explained a great many of the various 
difficulties which have arisen in connection with the Gotland section. 
Assuming, then, that the corals stood above the bottom and rose 
to or nearly to the surface of the water, it follows as a consequence that 
during the period of the development of the reef that it grew upward 
much more rapidly than the sediments accumulating around it. 
Later, however, when the reef reached the surface of the water its 
rapid growth ceased,? and sediments accumulated around it far more 
rapidly than it grew upward, the coral reef providing through its 
1 Personal communication. 
*This statement assumes that there was no sinking of the sea-bottom. If the sea-bottom 
were sinking, growth upward would continue until the downward movement ceased, after 
which the conditions described would obtain. 
