MOLLUSCA — CEPHALOPODA. 
171 
X vuriaas, to the siiiootli S. Goiqjdiauns. This again Gallery 
illustrates the cliange of ornament cliaracteristic of a 
declining series. Among the larger specimens the most 
noteworthy are Piizosia Ausletii, Pachydiscus pcramiylus, and 
Pachydiscus leptopliyllus. Some specimens clearly show the 
complex suture of this last, but the most interesting is the 
very large one from Eottingdean, 3 feet 8 inches in diameter ; Between 
the rapid increase in the width of the coiled cone may be WalLcases 
contrasted with the very slow increase in the large Lias 
specimen opposite. The largest known ammonite is P. scppen- 
radensis, from the Lower Senonian of AVestphalia, with a 
diameter of about 2 metres (0 ft. 8 in.). A plaster repro- 
duction of it is fixed at the north end of the Gallery. 
AV''e have already noticed the changes in ornament that 
characterise ammonite races as they advance to and recede 
from the acme of their development, and we have seen how 
the suture likewise becomes more complex and then returns 
to a simpler form. There is yet another and more obvious 
change. dust as the ascending stocks, beginning with 
straight form.s, gradually coiled the shell more and more 
closely, so, having reached their acme, they begin to uncoil 
and may ultimately return to a straight condition, if they 
do not previously become extinct. Further, instead of 
merely unwinding, they may lose the regularity of the coil 
aiul become wound in an asymmetrical spire or turret, like 
that of most gastropod shells. Already in Triassic times the 
Ceratites (using the term in a l>road sense) show all these 
retrogressive changes, ending in the straight Rh(d)dorcras. 
Of the various ammonite families that passed into Junissic 
and Cretaceous times, the Stepheoceratidae gave off a 
degenerate branch so early as the Bajocian Age. The 
eccentric Cado'inoccras has already been noticed, and here Table case 
are exhibited the further uncoiled Spiroceras hifurcciHm 8- 
and similar forms, which led to the straight Bacidina of 
the Callovian. 
It was, however, chiefly towards the close of the Table-eases 
Cretaceous Epoch that all the persisting races entered on 
this degeneration. Names have been given to the various ^ 
stages ot uncoiling, such as Crioccras, in which the whorls 
are partly separate (Fig. 96 a) ; Macroscaphites with the last 
whorl bent slightly back and then returning on itself 
(Fig. 96 rf); Scapyliites with a somewhat closer coil to start 
with and a more rapid return (Fig. 96/); Hamites, which 
starts with a small coil, then goes sti'aight for some distance. 
