16 REPORT — 1844. 



admitting it into the group. I think it well at once to premise, that the cha- 

 racters derived from the intimate structure of the shell are not likely to serve 

 for the distinction of species from each other, and that they will not often 

 distinguish genera ; but for the separation of some nalural families, I believe 

 that they will furnish the best single set of characters that the naturalist pos- 

 sesses, especially among particular groups, in which the application of other 

 characters is very uncertain. 



IX. Brachiopoda. 



36. The shells of the Brachiopoda or Palliobranchiata (Owen) present 

 many interesting objects for inquiry ; their structure is, in almost every in- 

 stance, quite distinct from that of the shells of the Lameliibrauchiate bivalves ; 

 so that, as I shall presently show, even amorphous fragments of shell maj' be 

 referred with certainty to this group, when not altered by metamorphic action. 

 I have recognized in the shells of Brachiopoda two leading types of con- 

 formation ; one of which is a peculiar variety of the plicated membranous 

 structure ; whilst the other is an equally peculiar form of the tubular. The 

 former occurs in the genus Terebratula and its allies, the latter in Lingula 

 and Orbicula. 



37. The shell of Terebratula psittacea, which (for a reason presently to be 

 specified) I shall take as a type of the first of these structures, is remarkable 

 for its divisibility into thin micaceous plates, which may be split into laminae 

 of extreme tenuity. I do not know any one of the Lamellibranchiate bivalves 

 whose shell corresponds with it in this respect, except Placuna and Anotnia, 

 which evidently verge towards the Brachiopoda. This facility of lamination 

 characterizes a large number of the fossil species of the group ; especially 

 those which correspond with the one now under consideration, in its peculiar 

 characters. The natural laminae thus obtained frequently aflbrd better sub- 

 jects for microscopical investigation than can be procured by making sections 

 in the ordinary manner. When these laminae are examined with a good 

 microscope, they are found to present a most remarkable and characteristic 

 appearance ; they are traversed by a very regular series of lines, usually 

 nearly straight, but sometimes slightly curved, and running quite parallel to 

 each other (figs. 27, 28). The distance of these lines from each other averages 

 about 1 -2000th of an inch, and from this average I have never found any very 

 wide departure, — the greatest distance I have met with being in Terebratula 

 octoplicata, where the space between them is about 1 -700th of an inch. 



38. When the broken extremities of these natural lamina are examined, it 

 is seen that the lines in question are produced by sharp foldings of the shelly 

 layer, which foldings are parallel to each other ; and this view is confirmed by 

 examination of the decalcified membi'ane, of which only one continuous stra- 

 tum exists in each lamina. 



39. When the natural internal surface of the shell is examined, a very 

 beautiful appearance is presented by it ; a most regular imbricated arrange- 

 ment is seen, exactly resembling a tiled roof, in which the lower margins of 

 the tiles are rounded, instead of being quadrangular (fig. 29). If a portion 

 of the surface be slightly rubbed down, so that the connection of these tile- 

 like markings with the interior structure can be traced, it is seen that they 

 are the extremities of the longitudinal folds just mentioned, each row of them 

 belonging to one lamina, and a series of these laminae cropping-out, one be- 

 neath another. 



40. When artificial sections, instead of the natural laminse or surfaces of 

 this shell, are examined, a great variety of appearances will be presented, ac- 

 cording to the mode in which the plane of the section traverses the plaited 



