INTIMATE STRUCTURE OF THE SHELLS OF BRACHIOPODA. 25 



distance from this. From the indistinctness of the breaks between the older and the 

 newer portions of the shell, both here and elsewhere, I am inclined to think that the 

 formative action must be more constant and gradual among the Brachiopoda, than it seems 

 to be among the Mollusca generally. 



Excluding the Discinida and LingulidcB, whose shells are almost entirely com- 

 posed of a horny animal substance, the shells of Brachiopoda may be said to be 

 almost exclusively calcareous in their composition ; the proportion of animal matter which 

 they contain, being far less than that which exists in the shells of most Lamellibranchiate 

 bivalves. When a portion of the shell of a recent Terebratula which has been preserved in 

 spirit, is submitted to the action of dilute acid, the greater part of it is speedily dissolved 

 with effervescence ; and in the membranous residue I have not been able to distinguish 

 with certainty and uniformity more than two layers, both of great tenuity, but one of 

 them considerably thicker than the other. This thicker layer, which is usually of a 

 yellowish hue, obviously covers the exterior in the manner of the periostracum of most 

 Lamellibranchiate shells, although more closely incorporated with its substance ; whilst 

 the thinner layer, which is quite pellucid and colourless, appears to have hned its interior, 

 this also being so closely incorporated with the shelly texture, as not to be separable from 

 it without the complete destruction of the latter. I have not been able to discern any 

 distinct structure in either of these membranes, although they exhibit faint markings, 

 which I am disposed to regard as nothing else than the impressions left upon them by 

 the peculiar shelly texture with which they are in contact. Intermediate layers are 

 occasionally to be met with, sometimes resembhng the inner membrane, and sometimes 

 the outer, in their characters. A repetition of the inner membranous layer may of course 

 be expected wherever a new layer of shell is formed within the old ; and I have found that 

 such a layer is sometimes furnished, in Crania, though not in Terebratula, with its own 

 periostracum. 



It is, however, in the texture of the substance which intervenes between these layers of 

 membrane, that the most characteristic peculiarity of the shells of the Brachiopoda exists. 

 In all the recent Terebratulida and JRhynconellidce, and in all the fossil specimens of those 

 groups, as well as of Spiriferida, Strophomenida, and Productidce, in which there is no 

 indication of metamorphic action, the shell is found to consist of flattened prisms, of 

 considerable length, arranged parallel to each other with great regularity, and at a very 



we substitute for the term " a layer of unpunctuated shell," " a layer of shell having minute punctuations." 

 When the uses of these passages (which are very different from what Mr. Sharpe supposed them to be) 

 come under discussion (p. 29), it will be seen to be very improbable that they should in any instance be 

 closed-up from the inside. No such statement can be justified, save by a careful microscopical examination 

 of a transparent section of the substance to which it relates, especially m fossil specimens, in which, from 

 the channels in the shell being filled with the matrix, their orifices (the punctuations) are often almost 

 indistinguishable, even when of considerable size (seep. 31). And all my own observations— ome of 

 them made upon specimens kindly supplied to me by Mr. Sharpe— lead me to believe that the passages 

 always retain their internal openings (as seen in PI. V, fig. 8), although these may be of very minute size. 



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