PHYLOaENY. 



127 



from the papers of Dr. Rowe (' Proc. Geol. Assoc.,' vols, xvi, xvii), and al.so 

 from notes furnished hv Dr. II. I'. IMackniore and Mr. T. H. Withers. 



THE PHYLOGENY OF THE CRETACEOUS ASTEROIDEA. 



If we examine the various species of a genus or group of related genera of 

 Cretaceous Asteroids, we find that there is a similar transition from smooth to 

 spinous forms tlirough an intermediate form, to that which has V)een ol)served in 

 Ammonites and Brachiopods. 



Both in Ammonites and Brachiopods single specimens sliow the whole life- 

 history of the individual, for the shell of the animal is luit materially altered in 

 character after it has once been formed. It is therefore possible to show, f. f/., 

 that the character of the ornament of the shell of an Ammonite was smooth 

 in its infancy, costate in its adolescence, spinous in the adult, and it has also been 

 shown that this life-history dei)icted by the individual is an ejiitome of the 

 phylogenetic history of the species (Buckman, ' Mon. Amnion. Inf. Oolite,' Pal. 

 Soc, 1905, p. cc). Similar ol)servations have been made with regard to Brachio- 

 pods (B)ickman, ' Quart. Journ. Geol. Soc.,' vol. Ixiii, 1907, p. 338) ; primitively the 

 Brachiopod .shell is smooth externally, more advanced forms ai-e j)rogi'essiveIy 

 costate and then spinous. Occasionally species may regress towards a primitive 



