28 G. LINDSTROM, ON THE SILURIAX GASTROPODA AND FTEROl'uDA OF GOTLAND. 



far as Klinteberg, but as they mostly are scarce there and freijuent in the north they 

 may be considered as North Gothand shells. 



Only five species are common to the strata over the whole island and conse- 

 quently the most characteristic species of all Gastropoda. These are Platyceras cornu- 

 tum, Tremanotus longitudinalis^ Pleurotoniaria nlnta, Oriostoma sculptum and Autodetus 

 cnlyptratus. 



With the exception of the minor faunal districts wliich have been sketched above, 

 such as Sandarfve and Djupvik, there are in other respects no obvious grounds for 

 discerning larger, faunistic divisions, as was attempted in my paper on the Brachio- 

 poda of Gotland ^) some years ago. There are so many (characteristic forms common, 

 such as Autodetus, Craspedostoma and Tremanotus. Moreover, it is generally the 

 case that distinctions based upon the occurrence of the species, are sooner or later 

 effaced in the same degree as the material for study increases. It is then evident, that 

 all such considerations have only a relative or transient value and properly speaking 

 are only valid for the time they were made, as each year brings new forms and mo- 

 difies older conclusions. New species are discovered and those formerly found are ob- 

 tained from localities and strata where they were unknown. 



As to the vertical distribution of the species as many as 29 have hitherto been 

 found exclusively in the lowest or shale beds, signed a on the synoptical lists. It 

 has not been possible, at present, to ascertain the succession of these species within 

 the shale beds or if there really be zones of vertical distribution within the range of 

 the more than hundred feet it comprises in thickness. In common with h it has besides 

 nine species and with both b and c 27 species and contains thus in all 65 species. 

 The next stratum b is nearly as rich, although having only twelve species of its 

 own, and in common with a nine species, common with both a and c 27 species and 

 with c alone sixteen species, thus in all 64 species. Richest of all is, however, the 

 third stratum c, with so many as 81 species peculiar to it, which in addition to 16 

 common with b and 27 species common with both a and b make up the sum total of 

 124 species. 



With regard to the occurrence of the 25 genera, of which tlie gastropodan fauna 

 consists, in the different strata, there are some, as Tryblidium, i'ah"eacuioea, Trochus, 

 Pycnomphalus, Craspedostoma, Holopea, Euchrysalis, Macrochilina and Onychochilus, 

 which are restricted exclusively to the highest strata. Some again, as Oriostoma, are 

 most abundantly represented in the uppermost strata, though there are also species of 

 it in the middle limestone and the shale. 



It may be questioned whether not any evidence is found of mutations in such 

 species, which occur in several strata, or in those, which have been found also in 

 younger or older formations. Such changes, also indicated through the presence of 

 nearly related species or varieties in older or younger formations, have most obviously 

 occurred in the following species, which are now enumerated. The matter is more 

 closely considered in the description of each species. 



^) Bidrag till kannedomeu om Gotlands Brachiopoder. Ofvers. Vet.-Akad:s Forhandl. 1860, p. 337. 



