SINUOUS EDGES OF TRANSVERSE PLATES. 347 
curve, into a variety of attenuated ramifications 
and undulating sutures. (See PL 38. and PI. 
37, Figs. 6, 8). Nothing can be more beau- 
tiful than the sinuous windings of these sutures 
in many species, at their union with the exterior 
shell; adorning it with a succession of most 
graceful forms, resembling festoons of foliage, 
and elegant embroidery. When these thin septa 
are converted into iron pyrites, their edges ap- 
pear like golden filigrane work, meandering 
amid the pellucid spar, that fills the chambers 
of the shell.* 
* The A. Heterophyllus, (PI. 38), is so called from the appa- 
rent occurrence of two different forms of foliage ; its laws of 
dentation are the same as in other Ammonites, but the ascending 
secondary saddles (PI. 38. S. S.) which, in all Ammonites are 
round, are in this species longer than ordinary, and catch attention 
more than the descending points of the lobes, (PI. 38. d. 1.) 
The figures of the edge of one transverse plate are repeated in 
each successive plate. The animal, as it enlarged its shell, thus 
leaving behind it a new chamber, more capacious than the 
last, so that the edges of the plates never interfere or become 
entangled. 
Although the pattern on the surface of this Ammonite is 
apparently so complicated, the number of transverse plates is 
but sixteen in one revolution of the shell ; in this, as in almost 
all other cases, the extreme beauty and elegance of the foliations 
result from the repetition, at regular intervals, of one symme- 
trical system of forms, viz. those presented by the external 
margin of a single transverse plate. No trace of these foliations 
is seen on the outer surface of the external shell. (See 
PI. 38, c.) 
The figures of A. obtusus, (PI. 35 and PI. 36), shew the rela- 
tions between the external shell and the internal transverse par- 
