DAVIDSON PAL^ONTOLGGICAL NOTES ON THE BRACHIOPODA. 109 



The only point remaining to be mentioned in connexion with this 

 valve, are the deep concave, often distinctly subspiral, depressions 

 visible in some species of Producta^ such as gigantea, and which 

 have been referred to labial appendages by the generality of authors. 

 They have 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 sufficient thickness, as they never 

 influenced the regular curve or convexity of the exterior of the valve. 

 In all the ProductidcE 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 Mollusca, 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 Terehralulidoe, Spiriferidoe, and Rhynchonellidce, as well as in 

 that singular group 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 Stropliomenldce 

 and Productidce 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 palaeontologist, and 

 should therefore be sought after with the greatest attention. 



In his Memoir, Mr. Hancock has stated that the brachial appendages sub- 

 serve at ORce the function of gills and of snstentation. 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 carried to the cirri, but more particularly to its 

 circulating through these latter organs, and to its return direct from them to the 

 heart." 



