DAVIDSON — I'ALiKONTOLOGICAL NOTES ON THE BKAClilOrODA. 100 
The only point remaining to be meutioned in comiexiou witli this 
valve, are the deep concave, often distinctly subspiral, deprcssious; 
visible in some species of Producta, such as P. gigantea, and which 
have been referred to labial appendages by the generality of authors. 
They liave been described by Mr. S. P. Woodward and myself as 
hollows probably occupied by the spiral arms ; for, if not, it would 
seem impossible to conjecture how they originated. Similar hollows 
could not, of course, be expected to be present in those species in 
which the shell did not possess a sufl&cient thickness, as they never 
influenced the regular curve or convexity of the exterior of the valve. 
In all the Produdidce we therefore find the muscles destined to open 
and close the valve complete. 
One of the most important features in connexion with Brachiopoda, 
and which has been made use of as a character in distinguishing 
them from other MoUusca, is the presence of those beautifully fringed 
appendages developed on either side of the mouth, to which the 
designation of " oral arms," or " brachial appendages," has been given 
by the greater number of naturalists.* 
In the Terehralulidce, Spiriferidae, and Ehynclionellidm, as well as in 
that singular gi'oup to which the term Davidsonidce has been pro- 
visionally applied, the oral arms are known to have been more or less 
supported by variously disposed and differently shaped testaceous 
appendages ; or, in other words, that the study of the animal of the 
existing species, composing the first and third families, has thrown 
much light upon the probable function as well as the manner in which 
these soft parts were attached and disposed. In the Stroi->liomenidoe 
and Pivdiictidce no such calcified supports have been hitherto detected ; 
and therefore we cannot speak with the same confidence as to the dis- 
positions which these parts assumed in the two last-named families. 
Every discovery that can throw some light upon the subject is of 
importance to the zoologist as well as to the palseontologist, and 
should therefore be sought after with the greatest attention. 
* In his Memoir, Mr. Hancock has stated that the bracliial appendages sub- 
serve at once the function of gills and of sustentation. To prove that they are 
aerating organs, " it is only necessary to refer to the manner in which the blood 
circles round the arms, and is earned to the cirri, but more particularly to its 
chculating through these latter organs, and to its return direct from them to the 
heart." 
