12 G. LINDSTRUM, ON THE SILURIAN GASTROPODA AND PTEROPODA OF GOTLAND. 



Martebo a peculiar, yellowish soft limestone near ]\Iyre, rich in Oriostomata and 

 their operciila. 



Fnrdkem and Linde. Some isolated hills as Sandarfve and Linde kullar, partially 

 consisting of crystalline, crinoidal limestone, partially a conglomerate of larger or 

 smaller pieces of corals and Stromatopora* and partly tine, earthy, red and gray lime- 

 stone beds. The last are remarkably rich in shells of Cephalopoda and Gastropoda. 



State of preservation. 



The state of preservation of the Gastropoda as well as that of other fossils in 

 these different rocks, is of course quite as variable as the nature of the rocks them- 

 selves and seems at first in several places quite unaccountable. So, for instance, the 

 shells of the Gastropoda and Lamellibranchiata are in many shale beds, as a rule, dis- 

 solved and present only as nuclei, while corals, trilobites and brachiopoda are well 

 preserved. Thus in one locality the shells are only represented as nuclei, as for in- 

 stance at Petesvik in Habblingbo, and again in a neighbouring locality, Djupvik in 

 Eksta, they are well preserved, with their shells. It may also be that many genera of 

 Gastropoda, as in the shale of VVisb}^ only occur as casts or nuclei, such as Subulites, 

 Loxonema, Murchisonia, Euomphalus, Bellerophon and Trenianotus, whilst in the same 

 bed others as Oriostoma. Pleurotomaria and Platyceras have retained their shell. In all 

 probability there is something in the condition of the chemical or mineralogical nature 

 of the shell which thus causes it to be dissolved in one group and preserved in the 

 other. In the upper limestone beds again by far the greatest number of the species 

 ai'e provided with their shell and all the delicate ornaments of its surface are visible. 



But the difficulty is to get them out entire and perfectly intact from the rock. 

 In such rich localities as Samsugn, Kyrkberget in ^^'isby many specimens can be col- 

 lected as they have been detached through the disintegration of the rock, which is 

 often crumbling to pieces on the surface. From the limestone of Sandarfve hill they 

 can be removed by carefully using the hammer and chisel. By delicate operations 

 with sharp needles they have been prepared out from almost all sorts of limestone 

 and been cleaned in greater perfection than could be expected at first sight. 

 In washing the fine, marly limestone mud, which is often found in fissures and 

 has probably originated through the decomposition of the limestone, many small Gastro- 

 poda, such as the MurchisonioB figured on plate XV have been obtained along with a 

 number of other interesting fossils. 



The few Gastropoda which have been found in the sandstone are remarkably 

 well preserved, showing the fine sculpture of their surface. 



In the majority of specimens the shell is entirely converted into calcareous spar, 

 but in a few others there are remains of the microscopic structure left. So for in- 

 stance in the genus Platyceras, in the common Platyceras cornutum, where at least 

 three different stata can be discerned. The Patellid Tryblidium has also the intimate 

 structure, to a certain extent, well preserved and also, on the inner side, the peculiar 

 horseshoe-shaped ring of muscular scars beautifully and most distinctly visible. In 



