252 E. H. L. Schwarz — Shell- Structure of Ammonites. 



is only one lamina thick, it becomes mucli more marked. Dotted 

 about the tangential section there are specks of conchiolin which 

 look as if they were tubules, but these are not seen in longitudinal 

 section and may be explained as places where two adjacent crystals 

 fail to interlock. In parallel polarized light, the tangential section 

 of the prisms, which show as tiny ovals, are not isotropic, proving 

 that they are not crystal prisms with basal planes; while in con- 

 vergent polarized light a beautiful monaxial cross is given (as is the 

 case also in the recent Nautilus), due probably to the aggregation 

 of organic and inorganic particles. 



In the longitudinal section of the extreme edge of the living- 

 chamber of Am. (Microceras) plnnicosta, Sow., Fig. 1 (p. 253), the 

 newly-formed prisms fan out slightly, producing a crinkling of the 

 surface, which afterwards disappears by the straightening out of 

 the prisms. This is due, undoubtedly, to growth by intussusception ; 

 not by means of actual canals as Koenigsborn ^ and others have 

 supposed, but rather by the shell-building material being passed on 

 through the conchiolin, just as water in the soil is passed on from 

 grain to grain, though no conduit is seen. Carpenter has also 

 shown that slight intussusception goes on in the Lamellibranch shell, 

 for he says that the boundaries of the prisms in Mya arenaria, L., 

 become obliterated in old age. 



The outer layer, corresponding to the porcellanous one of the 

 Nautilus, is usually taken to be represented by a clear crystalline 

 deposit external to the prismatic zone, as is shown in Am. plani- 

 costa figured by Hyatt.* But my section, Fig. 1, of the same, 

 from the same material, shows that this is nothing else than 

 part of the inner layer, which has split before the rest, and so 

 drawn in the re-crystallizing water, for the clear band is seen 

 dipping inwards and separating the true prismatic layer at the 

 edge. In another section. Fig. 2, showing the wall of the living- 

 chamber, the evidence of the origin of this outer layer is still 

 plainer. In most cases that I have examined the so-called outer 

 layer in like manner seems to be the outer part of the prismatic 

 layer re-crystallized ; it is, however, probable that a porcellanous 

 outer layer did exist, but, owing to its granules not being united 

 into a self-supporting tissue, it was quickly dissociated on the 

 death of the shell. Thus, in Fig. 1 the horny matter at the end 

 seems to have terminated such a layer which has gone, while iu 

 a Lias nodule containing Am. (Grammoceras) radians, Schloth., there 

 appears to be an actual granular layer (a), preserved between two 

 adjacent whorls, Fig. 3. Again, in Grioceras Callovlense, Morris, 

 there is a clear band of re-crystallized matter surrounding even 

 the extreme edges of the spines ; this is probably the representative 

 of the outer granular layer, for if it were part of the inner, one 

 would think that it would break through the bases of the spines. 



' Untersuch. ii. crust. Panzer ii. Molhisk. schaal., Berlin, 1877. Of course 

 I do not mean here that growth from Avithin can go on to the extent that Mery 

 postulated, Hist, de I'Acad. rog. des Sci., Mem., 1710. 



* Zoc. at. ■ ' , . 



