XIV 



YORKSHIRE TYPE AMMONITES 



Mar. 



though the ontogeny of an armaius Ammonite itself may only reveal 

 I, 2, 5*. The long, smooth stage (i) in young A. armaius may be 

 regarded as an example of cunctative palingenesis. The skip from 2 to 

 5*, the last 5* of the phylogenetic series, is saltative palingenesis. 



There are some other apparent exceptions to orderly development. 

 For instance, in the capricom group, instead of the costae accumulating 

 material at only one point, and so each one producing its tubercle, the 

 costae swell greatly all round : there is an exaggerated costate stage 

 which presently breaks up into the bituberculate stage. There is 

 something similar in the jamesoni-senes, where a unituberculate stage 

 passes into an exaggerated costate stage, which is not really a cata- 

 genetic development. 



Tachygenesis and Palingenesis (Saltative and Cunctative). — ^These 

 phenomena may be observed well in the capricom Ammonites and their 

 sphaerocone developments, the nautiliforms [A. nautiliformis, J. Buckman, 

 A. bechei, Sow., etc.). The capricom stage (serpenticone), with massive 

 annular ribs (really modified 5*), passes again into a sphaerocone stage, 

 with ornament 5** ; and there is constant shortening of the Capricorn 

 stage, earlier development of the nautiliform {tachygenesis), until, in the 

 latest species {A. bechei, etc.), the Capricorn stage is omitted altogether — 

 the Cymbites-siz.gQ (sphaerocone) passes directly into the nautiliform, or 

 secondary sphaerocone {saltative palingenesis). But the young whorls of 

 the nautiliforms retain a smooth sphaerocone stage longer than did the 

 capricoms — cunctative palingenesis. 



Then the capricorns themselves show saltative palingenesis. The 

 full development exhibited by their predecessor, A. planicosta, is i, 2, 3, 

 4 ; some capricorns omit stages 2, 3, and jump from i to the strong- 

 ribbed stage 4. 



The record of developmental stages of ornament among certain 

 genera of Arietidae illustrate tachygenesis and saltative and cunctative 

 palingenesis, as in the following table : — 



Table V — Development Stages (Ornament) 



