128 



FOSSIL ASTEROIDEA. 



plain form througli a costate phase. The progression or elaboration is known as 

 " anagenetic " development, and the retrogression as " catagenetic." 



It is regrettable that our present state of knowledge of Chalk Asteroids does 

 not allow us to recognise such definite phylogenetic series as those obtainable in 

 Brachiopods or Ammonites. The plates of an Asteroid are constantly being eaten 

 away and replaced by new calcareous matter, so that the adult plate may differ 

 considerably in character from its young phase. An opportunity for study, 

 however, is afforded by the fact that all the plates are not formed at once. The 

 more distal plates are younger than the proximal plates, and therefore resemble 

 more closely those of the young form. The resemblance is not, however, 

 quite exact, as they are formed later in the life of the individual, and may show 

 consequently characters which have appeared later in the history of the species. 

 Doubtless, if it were not for the paucity of the well-preserved specimens of 

 Cretaceous Asteroids much might be still made out by a comparative study along 

 these lines. 



The following paragraphs are only suggestions made in the hope that more 

 material may come to light at a future date. The great majority of starfishes are 

 and have been spinous forms, and I propose to assume that the original ancestor 

 in each group was spinous. 



Genera — Metopaster and Mitraster. 



(rugose) 



Metopasters. 



M. nneatus 

 (rugose) 



M. corniifiis 

 (spinous, with rugosities) 



M. Pnrl-insoni 

 (spinous) 



M. Hnnieri 



(spinous, with 



rugosities) 



MiTEASTEES. 



M. riigatKS 

 (rugose) 



M. cnmpadus 

 (smooth) 



Original spinous form. 



It is convenient in this gi'oup to consider the ornament on the raised central 

 area of the marginalia. It will be seen from the above diagram that the spinous 



