250 E. H. L. ScJmars. — ■ Shell- Structure of Ammonites. 



from Hanover.^ In the later rocks innumerable cases occur in 

 which the shells show their natural play of colour, such as the 

 Oxford-clay Ammonites, Am. (^Psiloceras) planorbis, Sow., from 

 Watchet, or Am. (Microceras) planicosta, Sow., from the Marston 

 Magna JVI^rble of Wiltshire. Hyatt ^ has figured sections of the 

 last with a short description, but he took the recent Nautilus as 

 a key to the understanding of the parts, therefore I think it worth 

 ■while to redescribe it with the help of a large series of sections of 

 other Ammonites. 



In the Ammonites and Nautili the inner layer appears to be 

 composed of fi.ne laminee of calcium carbonate and conchiolin, 

 alternating in var^'ing thicknesses, but always very fine; Blake ^ 

 gives -2-(rooo- to so^^o^o inch as the thickness of those of the recent 

 Nautilus, and the Ammonite laminse are about the same. These 

 laminae have been taken to constitute the primary structure of the 

 shell,* and this idea is helped by the fact that the shell tends to 

 split parallel to them, and thereby causes dissolution and re- 

 crystallization to follow the same lines, by sucking the water into 

 the cracks by capillary attraction; hence we see in the mineralized 

 shell the laminee reproduced. But under favourable conditions, as 

 when the shell is preserved in clay, such as is beautifully seen in 

 Am. (Ludwigia) opalinus, Kein., from Haresfield, this laminated 

 structure becomes obliterated, while the real primary structure is 

 preserved. This consists of innumerable prisms standing per- 

 pendicular to the surface, and united (probably, as we shall see 

 later, by the interlocking of their component crystals) into a self- 

 supporting tissue. These prisms also may be faithfully reproduced 

 in other minerals, as in the Blackdown Ammonites, Am. I^Scliloen- 

 bachia) varicosus, Sow., and Am. (Hoplites) denarius, Sow., where 

 the shell has a pink, faintly-iridescent hue, which may be con- 

 verted into marked iridescence by soaking chips in Canada balsam, 

 when they will show the prisms well, although they now consist of 

 silica; and also in the Gault Ammonites, e.g. Hoplites splendens, 

 Sow., where the pyritized prisms may be seen on looking at the 

 surface with a iinch objective by reflected light. In Am. (Ludioigia) 

 opalinus, Rein., Am. [Amaltheus) ibex, Quenst., Am. (Cosmoceras) 

 Jason, Eein., as they usually occur, the shell is white and opaque, 

 though otherwise unaltered ; when thin tangential sections of these 

 are soaked in Canada balsam they become perfectly iridescent, and 

 show unmistakably the prismatic- without the laminated-structure : 

 shells which have the latter only are incapable of regaining their 

 natural lustre. This deals the death-blow to the theory of iri- 

 descence caused by outcropping of the laminae in fine lines, as 

 illustrated by the celebrated Barton buttons ; or by means of 

 alternating films of different optical density, provided by the 

 calcium carbonate and conchiolin ; while it clearly indicates that 



1 F. von Eomer, Jahresber. Schlesisch Gesells., Breslau, 1873. 



2 Zoc. cii. * British Fossil Cephalopoda. 



* Carpenter, Brit. Assoc. Eeports, 1847, p. 46 ; Sorby, Pres. Address, Gaol. See, 

 1879, says the laminse are pierced by fine needles of aragonite, p. 31. 



