24 INTRODUCTION. 



margin (as seen at a a, hh, cc), as is the case with the outer layer of LamelHbranchiate 

 shells generally, whilst f/ieir inner layer, formed by the calcification of the basement-mem- 

 brane of the mantles, is augmented by the interposition of new lamina between the animal and 

 t/ie whole previous external surface of the shell, such new lamina being extended beyond the 

 preceding so long as the shell continues to grow, and bearing on its projecting part the 

 marginal addition which has been made to the external layer.^ I have occasionally met 

 with a second layer, in recent TerehratulcB, within the earlier-formed portion of the shell, 

 but confined to only a part of the surface, instead of extending beyond it. Such a layer 

 (represented at PL IV, fig. 1, a d^ is of precisely the same texture with the remainders 

 of the shell, and appears to be simply destined to afford a mechanical support which its 

 older portion may require, when this has been greatly extended by new growths. Several 

 such layers present themselves in the shell of Craiiia (PI. V, fig. 8), whose upper valve 

 is formed upon the plan of that of Patella, and is increased, both in thickness and diameter, 

 by new formations successively added to the internal surface, and extending beyond the 

 margins of the old. A like succession of layers, moreover, is easily to be distinguished in 

 the massive shells of many fossil Gasteropoda, in consequence of their ready separation 

 from one another ; but in these, as in the preceding cases, they are all repetitions of the 

 first or outermost layer, and do not present any of that difference in intimate texture which 

 almost invariably distinguishes the inner and outer layers in Lamellibranchiate shells. 

 Moreover, in all those Brachiopodous shells which are perforated by the vascular (?) canals 

 presently to be described, the layers are alike perforated by them, although the passages 

 are usually much smaller in the internal or last-formed layers than they are in the outer 

 and older, as is well seen in the portion of the shell of Terebratula delineated in PI. IV, 

 fig. 3. And when the passages themselves are large, as happens in the internal layers of 

 the shell of Crania (PI. V, fig. 8), their openings upon the inner surface are frequently 

 so small as to escape observation, when it is examined with a hand-magnifier only.^ 



The "lines of growth" which are so obvious on the external surfaces of the shells of 

 many Brachiopoda do not by any means constantly correspond with interruptions in 

 continuity of structure as displayed in vertical sections of the shell; thus in PL I, fig. 1, 

 there is seen at c such an inequahty of surface as appears to indicate a marginal addition, 

 yet the real extension, as marked by the line c c, appears to have taken place at some little 



1 Op. cit., 1847. 



2 It is apparently from observations of this kind, that Mr. Daniel Sharpe has stated, in his account of 

 the Genus Trematis (Quarterly Journal of the Geological Society, vol. iv, p. 67), that " an inner layer of 

 unpunctuated shell lining an outer punctuated layer, is a common occurrence among the Brachiopoda ; it 

 may be especially observed in the flat species of Ortliis, and in many Leptaense, covering all the central 

 parts of the shell, and leaving the punctuations open only round the margin. Among the Productse and 

 some species of Chonetes, the punctuations are closed up everywhere, except at the edge, by a gradual 

 deposit of shelly matter in the interior. In Crania, Thecidea, and some recent Terebrattdce, the punctu- 

 ations can only be seen in the interior round the edge of the shell ; but in the majority of the recent species 

 of Terebratula, they are equally visible over the whole shell." This statement will be perfectly correct, if 



