72 BULLETIN OF THE 



very much as Lytoceras fimbriatum and others are related to the more 

 involute Pliylloceras, and the different series contain more or less rep- 

 resentative or mimetic forms due to the resemblance occasioned by the 

 amount of the involution or the characteristics which are usually correla- 

 tive with the amount of involution. The differences between the series 

 are found in the development of the young, and the structural peculiari- 

 ties of the shell and septa. 



"When, however, the organization of the group no longer progresses, 

 but retrogrades by the uncoiling of the wborls in Scaphites Ancycloceras, 

 and Baculites, repeating — as shown by several authors, but notably by 

 Barrande — the earlier forms of the Nautiloids in inverse order, these, 

 though strictly mimetic, are produced by the encroachment of senile 

 characteristics. These are observed in the old age of such species as 

 Amm. Humphriesianus, where the old whorl becomes smaller, more 

 cylindrical, and if growth was continued, must eventually strike off 

 from the regular spiral as in Crioceras or.Lituites. This irregular- 

 ity is found at earlier and earlier stages of growth, and finally affects 

 the whole form as in the completely straightened Baculites. Direct 

 inheritance of senile characteristics is not claimed, but merely that the 

 retrogression of the individual in old age and the retrogression of the 

 group are similar, and both due probably to the same cause, exhaustion 

 of the powers of growth. 



There is, however, a notable exception, which can be accounted 

 for only by Professor Cope's law of "retardation," and which, to me, 

 was inexplicable until the appearance of his essay on the Origin of 

 Genera ; the two Gasteropod-like genera, Turrilites among Ammonoids, 

 and Trochoceras among Nautiloids. With regard to the latter, there 

 are no certain data ; but the former are produced at first in varieties of 

 species, which have, according to Quenstedt's, Oppel's, and my observa- 

 tions, simply prolonged into the adult an individual variation common 

 enough in the young shells. The young of several species of typical 

 Ammonites often assume the spiral, although this is entirely suppressed 

 at a later stage, and the succeeding whorls resume the normal mode 

 of growth and revolve in the same plane. When, therefore, the 

 normal mode of development is " retarded," we find even in the adult 

 this Turrilites-like condition of the young, which is as truly reversional 

 as the Orthoceratitic young of Goniatites fecundus. This happens 

 occasionally in the lower Jura, and finally, after the progressive stage 



