34 GENESIS OF THE ARIETID. 
Coroniceras, and Asteroceras, the stages of decline in individuals are not usually 
attended by such complete metamorphoses in these normal progressive forms 
as in Oxynoticeras and many others among the jurassic Ammonitine. The 
keel persisted, and is generally still visible even in extreme age, though often 
greatly reduced in size; the whorl also usually continued the same rate of 
growth so far as dorso-abdominal diameter is concerned. It therefore appeared 
to increase in size throughout life when viewed laterally, even in very large 
individuals of Vermiceras, Coroniceras, and Asteroceras. It is to be anticipated, 
"in some very rare cases of exceptional longevity even in these species, that the 
keel would be absent and the whorl would become rounded. This happened in 
senile specimens of Oayn. Lotharingum, which apparently possessed less power 
of resisting the effects of extreme age. This is a very interesting fact, since 
Oxynoticeras is the paracmic series of the Arietide, and according to our views 
would be likely to exhibit strongly marked degradational characters. 
It is obvious that the decrease in the size of the whorl, if continued long 
enough in old age, must have finally caused the last whorl to strike off from 
the regular line of the spiral, as in the Crioceras. We searched the collections 
of Europe during the year 1873 for a specimen of a normal species of large 
size and sufficiently advanced in age to refute or confirm this view, and finally 
found one, through the aid of Professor Mésch, in the Museum under his charge 
at Zirich. This was a large fossil of the Neocomian from Sentis, in which the 
adult whorls were ribbed, but the outer whorl old, smooth, and contracted to such 
an extent that at a short distance from its termination it was separated by a 
distinct gap from the abdomen of the next inner whorl. Professor Mésch was 
impressed by this fact, and gave this specimen, which he considered a new 
species, the manuscript name of Amm. (Scaphites) umbilicus, which it probably 
still retains. 
I have examined all the figures of M. Barrande, anticipating the finding of 
some marks of senility in individuals among the lower types of Nautiloids, and 
have not been disappointed. M. Barrande classifies the form of the siphon as 
“the cylindrical, the nummuloid, and the mixed”; and though he nowhere, 
so far as I can find, describes these metamorphoses of the siphon as stages of 
development, yet this was probably his real view, since in all his figures, suffi- 
ciently complete to show the young, when the siphon is nummuloidal in the 
adult it is cylindrical in the young.' Barrande’s figures also exhibit clearly 
the degradation of the nummuloid siphon, and its return during old age to the 
cylindrical form;? but I cannot find that this eminent author regarded these 
metamorphoses as having been caused by senility. 
1 Phrag. simples, pl. xix. fig. 9, Gomph. Belloti, pl. xxxti. fig. 6, Phrag. perversum, var. subrecta, pl. ¢. 
fig. 11-17, Cyrt. Logani, pl. clxxxii. fig. 2-10, and Cyrt. indomitum, pl. clxiii. fig. 5, all show the development 
of the nummuloid siphon from the smaller cylindrical tube of the young, or else it has a lessened diameter 
approximating to the cylindrical condition in the young. 
2 Cyrt. rebelle, pl. elxiv. fig. 7, exhibits the change during growth of the siphon, which transforms it 
from a nummuloidal to a cylindrical tube, and causes the shifting of the position from close proximity to 
the convex side to near the centre of the last formed septum. Orthoc. docens, pl. ccl. fig. 7, exhibits a 
similar series of metamorphoses, but the siphon remains at the centre of the whorl. 
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