32 G. LINDSTRÖM, ON THE SILUIUAN GASTROPODA AND PTEROPODA OF GOTLAND. 



their lainellar edges, often uneven and at iri'egular distances froin each otlier, there 

 are tine and elevated, as it were, ornamental liries. These do not always inni parallel 

 to the lines of growth, but cross thein obliquely in soine species, as Pleurotoraaria ex- 

 quisita and Onychochilus cochleatura. These elevated lines of varying size are in 

 several species replaced by sunk and narrow grooves. It would then be proper to 

 distinguish between lines of growth and ornamental lines, which are formed indepen- 

 dently of the former or coördinate to them. 



Through the presence of certain genera the following conclusions may be drawn 

 as to the character of the fauna, of which they formed an ingredient and consequently 

 of the natural conditions of the sea in which the including strata were deposited. 



1. The fauna of the silurian heds of Gotland was a littoral fauna. Snch genera 

 as Chelodes and Tryblidium make this conclusion valid. The Chitonidse, to which 

 Chelodes belongs, liva now generally down to a depth of 25 fathoms, though some 

 small ones have been found exceptionally at 100 fms. Tryblidium is a nember of the 

 group of the Patellidas. These are in a still higher degree shallow water shells, living 

 near the shore on seaweeds and rocks, and the occurrence of the Tryblidia is sugge- 

 stive of the former presence of algce, on which they might have lived, though no traces 

 of these are left. It has been said ^) that the Upper Silurian shales of England through 

 their fossils and the nature of the shale itself make it evident that they have been 

 formed in a deep sea. Since the explorations of the låter years the range of the deep 

 sea region upwards cannot be considered higher than 400 fathoms. But already at a less 

 considerable depth there reigns, as has been shown, an almost absolute stillness in the 

 water and the motion of the surface cannot infiuence the bottom at a depth below 

 40 fms. Occasionally, at more exposed coast lines, as near New Foundland the bottom 

 has been disturbed to a still greater depth, when violent tempests have raged, but 

 this is of course exceptional and the traces of the disturbance must again, during the 

 otherwise prevailing calm be effaced through new sediment. 



In consequence of what has been advanced above, shells imbedded at greater 

 depths could not show signs of having been subjected to any abrading motion of 

 the waves. Now, the fossils enclosed in the shale beds of Gotland on the contrary 

 attest in many instances by their abraded condition that they have been tossedabout 

 by the waves and, consequently, have not been imbedded in a bottom of any consi- 

 derable depth. The great raovement in the sea, where they lived, is also evident through 

 the shells sometimes having been broken and then mended. Thus there are specimens 

 of Pleurotomaria labrosa from the shale of Westergarn, showing such a break in the 

 body whorl, which had again been repaired during the lifetime of the animal. The 

 superincumbent limestone strata are for the most part conglomerates of corals and other 

 fossils, and, when fine grained, the limestone consists entirely of a sort of calcareous sand, 

 the grains of which are comminuted shells and corals. It thus highly resembles 

 the limestone, which is forming now a days out of similar material along the shores 

 or on the beaches of the tropical regions for instance near the Westindian Islands. 



') FucHS, Welche Ablagerungen haben wir als Tiefseebilduugen zu betrachten? in »Neues Jahrbuch fur 

 raineralogie etc», II Beil. Bd. 1883, p. 565. 



