THEORY OF RADICALS AND MORPHOLOGICAL EQUIVALENCE. 30 
Neumayr was the first to trace systematically the uncoiled forms of the 
Cretaceous to several groups and distinct species of normal close-coiled Am- 
monitine,’ thus confirming Pictet’s isolated but suggestive observations on 
Acanthoceras angulicostatum, and declared that Acanthoceras, Olcostephanus, and 
Hoplites were the radicals of the uncoiled forms previously included under a 
number of generic names by D’Orbigny and other authors, and also traced the 
genera Hamites and Turrilites to an origin in Lytoceras. Uhlig? holds views 
similar to Neumayr, tracing most of the Crioceratites to Hoplites, but con- 
siders that Olcostephanus, Acanthoceras, and Aspidoceras had also crioceratitic 
derivatives. Our conclusions, therefore, are in accord with the results of the 
researches of Quenstedt, Neumayr, and Uhlig upon the same class of forms. 
The Ammonitinz of the Lias and Qélites in extreme old age, as a rule, lost 
the tubercles, pile, and often the keel, the whorls became smooth, and decreased 
in size, tending to take on a rounded or triangular outline in section, according 
to the group in which the species belongs. Thus, in Plate I, Fig. 24 and 25 
show the changes which took place in the old whorl of Cal. raricostatum, and Fig. 1 
and 2 the similar effects in the old age of Cul. carusense. The whorl in both be- 
came rounded, and lost its ribs, ete. On Plate V. Fig. 8, 9, may be seen the old age 
of Cor, Gmuendense, and on Plate VI. Fig. 1, 2, the similar old age metamorphoses 
of Cor. trigonatum. In these last the quadragonal whorl of the adult became tri- 
gonal, instead of being rounded by senile degradation as in the first instance. 
On Plate X. Fig. 1-3, there are illustrations of individuals of Asf. obtusum, which 
can only be accounted for as the results of premature old age in this species, 
since the young until a late period of growth are identical, and both Professor 
Fraas of Stuttgardt and the author have identified them under this name. 
These. changes are all due to the loss of power in the old animal, which 
can no longer maintain its normal rate of increase in size as it grows. Thus 
the smaller discoidal whorls of Caloceras with low keels became rounded, and 
the quadragonal whorls of the keeled and channelled Coroniceras became tri- 
gonal. The latter is really a stage of reduction in size, intermediate between 
the quadragonal form and completely degenerate rounded whorl of extreme 
age, but, so far as is now known, this last stage was not reached by the progres- 
sive forms of the Arietidse except in Oxynoticeras.* 
Notwithstanding the evident loss of power, and the consequent and well 
marked changes taking place in the old whorl of many species of Vermiceras, 
1 Zeits. deutsch. geol. Gesell., 1875, pp. 874, 875, 924, 935, and subsequently with Uhlig in Paleon- 
togr., X XVII. 
? Denksch. Acad. Wien, 1883, XLVI. p. 258. 
® One of the finest illustrations of the effects of senility upon the shells of the Ammonitine of the 
Jura is given by Waagen (Geol. Surv. of India, Ceph., Jurass. Fauna von Kutch, pl. xi. fig. 1). In a 
large specimen of Perisphinctes aberrans, the old-whorl became smooth, greatly reduced in size, rounded, 
less involute, and finally exhibited a series of heavy folds on the sides. These senile folds are also common 
in the old of many forms of the Upper Jura, but are rarer and smaller in the Oélites and Lias. They may 
be due to prolonged arrests of growth, and the decline of the power to resorb the thickened edges of the 
apertures, after each period of rest. But, whatever the cause, they certainly indicated a loss, not an in- 
crease of strength. This is shown by the degradation in size and form of the whorls in such examples 
as are given below in the descriptions of Cor. Bucklandi, Cor. trigonatum, Ast. obtusum, and especially Oxyn. 
Lotharingum. 
5 
