34 GENESIS OF THE ARIETIDiE. 



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 Ammonitinae. 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 Oxyn. Lotharingum, which apparently possessed less power 

 of resisting the effects of extreme age. This is a very interesting fact, since 

 Oxynoticeras is the paracolic series of the Arietidte, 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 Mosch, in the Museum under his charge 

 at Zurich. 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 Mosch 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. 



1 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. 1 Barrande's figures also exhibit clearly 

 the degradation of the nummuloid siphon, and its return during old age to the 

 cylindrical form; 2 but I cannot find that this eminent author regarded these 

 metamorphoses as having been caused by senility. 



1 Phrag. simplex, pi. xix. fig. 9, Gomjih. Belloti, pi. xxxii. fig. 6, Phrag. perversum, var. suhrecla, pi. c. 

 fig. 11-17. Cyrt. Logani, pi. clxxxii. fig. 2-10, and Cyrt. indomilum, pi. 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, pi. clxiv. 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, pi. ccl. fig. 7, exhibits a 

 similar series of metamorphoses, but the siphon remains at the centre of the whorl. 



