1 88 PR f MAR Y FA CTORS OF OR CAN 1C E VOL UTION, 



The shells of the Ammonitinae, however, are of the 

 fourth and fifth kinds almost exclusively, and in fol- 

 lowing out the separate genetic series one has to dis- 

 tinguish the progressive gradations by means of the 

 greater or less amount of involution even in the Go- 

 niatitinae of the Devonian and Carboniferous. 



"There are also some very remarkable facts show- 

 ing that the coiling is closer in the Mesozoic than in the 

 Paleozoic forms as a matter of hereditary derivation. 

 The young of the Silurian and Devonian forms have 

 the open, slowly coiling whorls figured by Sandberger 

 and Barrande and repeated by myself as referred to 

 above, but the young of all Mesozoic forms are close 

 coiled so far as known. This is shown in the centre 

 of the umbilicus by the perforation or central opening 

 which is extremely large in most of the Paleozoic Go- 

 nititinae but becomes almost obliterated in the true 

 Ammonitinae of the Jura. 



"In tracing parallels between development of the 

 individual and the series among Ammonitinae it has 

 been found by Branco and the author, that in orna- 

 mented shells the young are first like a nautilus in the 

 sutures, then have a goniatitic stage like the first rep- 

 resentatives of the order of ammonoids in the Paleo- 

 zoic, and that during these stages it is invariably 

 smooth and similar in general form to these same an- 

 cestors. After this nepionic stage is passed through 

 the sutures and the characteristics alter with greater 

 or less rapidity, but the stages show decisive parallel- 

 isms with the immediate ancestors of the same genetic 

 series. Some of the best examples of palingenetic de- 

 velopment of this kind, where the later stages of growth 

 present parallels with proximate ancestors, are cited in 



