16 PROCEEDINGS OF THE 



bird differs from the nearly allied wild species in having a largely 

 developed dewlap extending from the base of the under mandible 

 down the fore part of the neck to its base. It cannot yet be 

 said to be a settled question as to the precise original stock from 

 which the valuable barndoor breeds have descended. 



III. — Notes on the genera of extiiict fossil shells, Bellerophon and 

 Porcellia; their classification amongst the Molluscay and their 

 distribution in the Silurian and Carhoniferous strata of the West 

 of Scotland. By Mr John Young, RG.S. 



Mr Young stated that at one time this interesting group of 

 shells had been placed by palaeontologists among the Cephalopoda, 

 the highest division of the Mollusca, and regarded as fossil repre- 

 sentatives of the recent Argonautidce, which possess a symmetrically 

 coiled shell as in Bellerophon and Porcellia, but like them not 

 chambered, as in the genus Nautilus. In the more recent classifi- 

 cation of the Mollusca, Bellerophon and Porcellia are now placed 

 among the Gasteropoda in that division termed the Nucleohranchiatcc, 

 which consists of entirely pelagic animals, some having shells, others 

 none, and, according to Woodward, swimming at the surface instead 

 of creeping on the bed of the sea. Prof. Owen believes, however, 

 that from the thickness of the shells in many of the species of 

 Bellerophon, they may have been adapted to protect their owner 

 while crawling over the sea bottom ; for it can scarcely be insisted, 

 he says, that all were necessarily floaters on account of their 

 organisation. In recent seas the extinct genera are represented 

 by the genus Atlanta and the sub-genus Oxy gyrus. In palaeozoic 

 times, the genus Bellerophon commenced its existence, so far as is 

 known, in the lower Silurian period, and became extinct in the 

 Carboniferous ; it is represented over the world by about seventy 

 species, fourteen of which are found in Western Scotland, viz., four 

 in the Silurian rocks of the Girvan valley, and ten in the Carbon- 

 iferous limestone strata of the district around Glasgow : the most 

 abundant and characteristic Carboniferous species being, B. Urii 

 and B. decussatus, and their varieties. The rarer genus Porcellia 

 ranges from the Devonian to the Triassic period ; ten species have 

 been found, but only one of these, P. armata, has yet been 

 discovered in the Glasgow district. 



