THEOBY OF RADICALS AND MORPHOLOGICAL EQUIVALENCE. 33 



Neumayr was the first to trace systematically the uncoiled forms of the 

 Cretaceous to several groups and distinct species of normal close-coiled Ara- 

 monitinae, 1 thus confirming Pictet's isolated but suggestive observations on 

 Acanthoceras angulicostatiim, 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 2 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 Ammonitinoe of the Lias and Oolites in extreme old age, as a rule, lost 

 the tubercles, pilse, 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 ofCal. raricostatinn, and Fig. 1 

 and 2 the similar effects in the old age of Col. caruscnse. The whorl in both be- 

 came rounded, and lost its ribs, etc. 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 Ast. 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 Arietidaa except in Oxynoticeras. 3 



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., XXVII. 



2 Denksch. Acad. Wien, 1883, XL VI. p. 258. 



3 One of the finest illustrations of the effects of senility upon the shells of the Ammonitinae of the 

 Jura is given by Waagen (Geol. Surv. of India, Ceph., Jurass. Fauna von Kutch, pi. 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 Oolites 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. 



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