76 



GENESIS OF THE ARIETID.E. 



in any other species of this genus. Thus a high degree of specialization in the 

 development of keel, channels, and pilae is correlative with decidedly retrogressive 



changes. 



In Coroniceras, the Bucklandi series exhibits very decided changes in both 

 the individuals and the species. The tubercles were first lost during the old age 

 of the individual, the sides became more convergent even in Bucklandi itself, the 

 abdomen narrower, the pilae reduced to folds and bent like those of the adult of 

 Ast. oUusum, the channels shallow and finally almost obsolete, and the keel, even 

 though becoming apparently more prominent on account of the convergence of 

 the sides and obsolescence of the channels, was really not so sharp or well 

 defined. 



Cor. orbicidahtm exaggerates all these old age changes, becoming narrower on 

 the abdomen, with more convergent sides, and this convergency began even 

 in the ephebolic period in some examples. Similar changes occurred very late 

 in the life of individuals in the next subseries. Thus even the convergency of 

 the sides was not found in the adults of Cor. rotiforme in many specimens, and is 

 but. slightly developed even in the extreme old age of some of this species, and in 

 its predecessor, Cor. kridion. This characteristic is, however, observable habitually 

 in the adults of Cor. lyra. These lead into Cor. Gmuendense of the same series, 

 wdiich had very convergent sides in the adult, and was often also destitute of 

 tubercles. The last were confined to the earlier stages of this species, and in old 

 ago the changes were very marked and rapid. The extreme variety of Cor. 

 trigonaium inherited convergent sides, smooth and half obsolete pila?, narrow 

 abdomen, shallow channels, and elevated keel, so early that we may say with 

 confidence they all appeared in the ephebolic period. 



The old whorls of Cor. Gmuendense and Cor. trjgonatum 1 have the sides of the 

 whorls convergent and a decidedly trigonal form. This form is correlated with 

 obsolescing pilae and a marked though late decrease in the sutures. These 

 lose the characteristic prominence of the second lateral saddle, which is a pro- 

 gressive characteristic in this genus. All the lobes and saddles also become 

 broader and decrease in proportionate length, and finally in extreme age the 

 abdominal lobe is decidedly shortened. 2 In the Museum of Comparative Zoology 

 there was also a much smaller specimen, 3 in which the same stage of decline had 

 been reached at an earlier age. The law of succession was, therefore, quite differ- 

 ent from that which governed the inheritance of progressive forms. The most 

 retrogressive of the bucklandian varieties were those which were most closely 

 connected in every way with Cor. orbkulatum. The genetic connections also 

 between Cor. rotiforme and Cor. lyra were traceable only through those varieties of 

 rotiforme which had the most convergent sides and the most retrogressive pila?, 

 tubercles, etc. This also holds for the connections between this last and Cor. 

 Gmuendense and Cor. trigonatum. 



The law of succession in catagenesis, therefore, is that retrogressive species in each 

 separate genetic series are the direct descendants of retrogressive varieties or forms. The 

 facts consequently are in strict accord with the theory of descent with modification. The law 



1 PI. v. fig. 8, 9; pi. vi. fig. 3; pi. vii. fig. 1. L\ v ii. fig. 1. a pi vi g g \ o 



