126 FOSSIL ASTEROIDEA. 



GENERAL CHARACTERISTICS OF CRETACEOUS ASTEROIDEA AND 



OPHIUROIDEA. 



The majority of Cretaceous starfishes belong to the Phanerozonate forms 

 included in the families Pentagonastef idse and Pentacerotidae. Modern forms of 

 the genera of these families are widely distributed geographically, but, generally 

 speaking, they are characteristic of warmer waters than those of the English 

 Channel of to-day. 



The Chalk starfishes are specialised types which, although approximating to, 

 are not identical with, modern genera. The differences, at any rate in some cases, 

 appear to be distinctly physiologically advantageous. 



Metopaster and Mitraster, the most abundant of all Chalk starfishes, possess 

 not only a specialised type of ornament but also characteristic massive plates. 

 The arms tend to become shortened and the disc correspondingly enlarged. 



The Chalk species of the Pentacerotidas differ from the modern forms, 

 inasmuch as they are more strongly built ; the abactinal areas are not reticulate, 

 and all species possess intermarginalia which cause the characteristically deep 

 body of these forms. 



The Chalk is a deposit formed in seas which were sufficiently distant from land 

 to avoid any great admixture of clay or sand. Qlobigerina and other forms of 

 pelagic Foraminif era floated in abundance on the surface of the sea, which, because 

 of its temperature, must have been exceedingly favourable to prolific forms. 

 In the circumstances there must have been an abundance of food for starfishes, 

 and we find, therefore, that the long-armed, comparatively active Astropectinidae, 

 which were so characteristic of the Jurassic shallow water deposits, are displaced 

 by more sedentary forms which tend to specialise, so as to obtain, by the enlarge- 

 ment of the disc or development of intermarginalia, the largest possible space for 

 their digestive organs. 



The irregular Echinoids which are so characteristic of the Cretaceous seas are 

 similarly sedentary forms. 



The fossil Ophiuroidea also closely resemble modern forms. The isolated 

 vertebral ossicles of 0. serrata, figured PI. XXVII, figs. 3 c, 3 d, 3 e, cannot be 

 distinguished from the ossicles of recent Ophiuroids. Complete specimens of 

 Ophiuroidea and Asteroidea are rare, but isolated plates are very numerous in 

 the Upper Chalk. They are, on the contrary, rare in the Lower Chalk, 

 according to experienced collectors, as, for example, Mr. Dibley. 



The following starfishes are found in the zones indicated. The list is compiled 



