I'lli: THREE M0DE8 "1 DEVELOPMEN I 



The accomplished author of U Pallontologie Francaise" denied thai the 



internal pai cted 1>\ * »1 < I age, u ne montrenl qu'une complication 



toujoii • jamais de d ge"ner< Tin- error was corrected 1>\ 



•-•ilt who pointed <>ut thai 1 1 1 » * closer approximation of the Butures in large 

 individuals was due t<> senility, and the author is now able to record thai he has 

 either observed or Been Bgured similar cases oi approximate Buturea, indicating 

 senile degradation in all of the different forms of chambered shells, except in 

 Belemnoids, which have nol yel been investigated. 



D'Oi »igny and Quenstedl were both satisfied with noting the details of 



the ol i tin- individual, the diseased aspect of certain forms, and their 



luction of the characters of their own young and of those of older forma, 



but did not attempt to explain the wider meaning of these parallelisms. In 



form.: re have asserted that the close similarity between the smooth, 



the Cretaceous, the extreme nostologic form oftheAmmo- 



and the smooth, straight Orthoceras ol the Cambrian, the comi i radical 



from which both Ammonoids and Nautiloids Bprang, is parallel with the resem- 

 blances w bich exist between the nostologic or oldest Btages of the individual ami 

 i \ oung 

 This resemblance between radical ami geratologous forms in smaller groups, 

 like tin- ArietidfB, was Blighter, ami often consisted merely in the smoothness 

 of the shell, or loss of tin- keel, or decrease in the amount of involution of 

 the whorl. I ' /' planorbe both have the compressed helmet- 



I outline of tin- whorl in section, and are Bmooth, though CoUeimti exag- 

 m, or i^ more involute and Matter than planorbe. Tin' most 

 version in the Lias are found in Oxynoi 

 Lotkaruigum. in which the old whorl loses its keel, and exactly reassumes through 

 n pressed helmet-shaped aspect of the adults of Ptil. pla 



Even this extren xample among ArietidsB, however, is not in anj sense 



an uncoiled -hell. It i- very nearly a complete parallel with the smooth 

 I form Psiloceras, ami may therefore be termed a noso- 

 lt barely attains this extreme rank in degeneration, whereas other 

 • . iiii-h retain the keel and do not decrease in - 

 whorl, are only clinolo^ic npproximationa 



mblances which occur between the young and old of the same indi- 

 vidual in the same parts and organs take place because th gnns lose their 



the functions which distinguished them in the adult, and 

 ter partlj or wholly atrophied and resorbed. 



The 'I'm. i Mo - "i l»i \ 1 1 oral m. 



The likeness between the youn rowth and the senile bI 



line in tin- same individual i- as wc have just shown, due to the disap- 



eed ephebolic chai red in the 



and adult iwth In groups the resemblances 



dical and milar suppression in 



