NATURAL HISTORY SOCIETY OF GLASGOW. 39 



the millstone grit, the whole representing a group of strata some 

 3000 feet in vertical thickness, it is possible that some of the ^ 

 varieties may have been evolved from earlier types, during the 

 very long period of the accumulation of these strata. Although 

 Sjnrifera trigonalis, with its var., S. hisulcata, are not known from 

 any strata earlier than the Carboniferous period, yet it probably 

 existed over some tract of the sea-bottom during pre-Carboniferous 

 times. 



I have already stated that in the Old Eed Sandstone period, 

 and in that of the calciferous sandstone series of the west of 

 Scotland, we have no evidence from the contained fossils that the 

 sea occupied the tracts where these strata were being deposited; 

 but after the outpouring of the volcanic rocks forming our 

 trappean hills, there appears to have been some change in the 

 physical features of the country, the sea having apparently had 

 ready access over the area as often as the land was depressed under 

 its level. Mr Robert Craig of Beith, in his interesting paper " On 

 the first appearance of certain fossils in the Carboniferous strata 

 around Beith and Dairy,"* points out that, in the strata overlying 

 the lowest of the volcanic series of that district, and which belong 

 to the calciferous sandstone period, there is a bed of coal represent- 

 ing an old land surface, and that over it, in a bed of argillaceous 

 shale, the first of the group of marine organisms make their 

 appearance. These appear in such force, and are represented by 

 so many genera and species, as to lead me to believe that they must 

 have migrated from some other tract of the sea bottom outside 

 the area of the west of Scotland, where they had been flourishing, 

 either in pre-Carboniferous times, or in the period represented by 

 the fresh- water strata of the calciferous sandstone group. In this 

 bed, Mr Craig has noted sixty species of molluscs, besides corals 

 and crinoids; while amongst the Brachiopoda Sjnrifera trigonalis, 

 and some of its varieties, make their first appearance. I have 

 found the shell, on nearly equally low horizons of strata, in other 

 portions of the coal-field around Glasgow, and it has also been 

 obtained from strata of the same age in other districts of Scotland, 

 as well as in England, Ireland, and other European countries 

 where marine Carboniferous strata prevail. 



What was the ancestor, in pre-Carboniferous periods, of S]jmfera 

 trif/onalis and its varieties we do not know. Mr Davidson, the 

 * Trans. Geo!. Soc, Glas., VoL v., p. 36. 



