94 Proceedings of the Royal rhysical Society. 



growth to {ci) rapid oscillations in tlie bed of the Carboniferous 

 sea, or seas, resulting in a sudden increase or decrease of 

 depth; (b) change of habitat, brought about by unlooked- 

 for irruptions of muddy sediment on a sea bottom peopled 

 with a molluscan fauna accustomed to purer conditions of 

 life ;* (c) the necessity for change of food from that on which 

 the species had been accustomed to subsist in more congenial 

 areas ; {d) changes in climate. 



One fact, which tends to confirm the view that irruptions 

 of sediment have been one of the causes at work, is worthy 

 of notice. Mr A. Sommervail has pointed out that wherever 

 the limestones thicken out, or one or more bands come to- 

 gether, there the Brachiopoda become of larger size. 



Although the Scotch Carboniferous Mollusca are, as a rule, 

 smaller than their brethren of other Carboniferous areas, their 

 state of preservation is often quite as good, and sometimes 

 superior to the latter. The number of individuals does not 

 appear to have been diminished, but growth simply retarded, 

 whilst the species are certainly less in number than in the 

 other rocks of Carboniferous age. 



I. The History of the Chough (Fregilus graculus) in Scotland. 

 By J. Hamilton Buchanan, Esq. 



(Read 15th March 1882.) 



Throughout successive ages there have been constant 

 changes going on in the relative numbers of the animal king- 

 dom. Many species which were once common have now 

 become scarce or extinct, and others which were formerly 

 extremely rare are now abundant. In the beginning of the 

 present century, the chough was certainly much more widely 

 and abundantly distributed in Scotland^than at present ; and, 

 as its range is perhaps becoming narrower with each succeed- 

 ing year, I venture to offer the following notes on its past 

 distribution to the Society. In collecting my information, 

 I have not confined myself strictly to Scotland, but purpose 



* S{»e Armstrong and Young, Cat,, 1871, p. ;^*i. 



