130 



FOSSIL ASTEROTDEA. 



Genus — Staurandebaster.. 



8. argus 8. ocellatus 

 (spinous) (spinous) 



»S'. squamatvs 

 (smooth) 



S. pistilliferus 

 (smooth) 



S. coronatus 

 (smooth) 



8. Boysii 

 (spinous) 



S. bipunctatus 8. bnlbiferus 

 (spinous) (spinous) 



Original spinous form. 



The left-hand stem and branches of the diagram above are occupied by the 

 generalised species which have long tapering arms. They show the transition 

 from spinous to smooth forms. The spinous form of these species is 8. Boysii. 

 In the lower zones of the Chalk one species (8. coronatus) appears. This has 

 neither spines nor rugosities on the majority of its marginalia, although a few 

 distal marginalia are rugose. 8. squamatus, which is almost identical in character 

 with 8. coronatus, except that it is of smaller size, and 8. pistilliferus (arms not 

 known) are the smooth forms which characterise the upper or middle zones of 

 the Chalk. 



The right-hand stem and branches are occupied by various specialised forms 

 from the upper and middle zones of the Chalk. 8. bulbiferus shows a specialisa- 

 tion in the bulbiform character of the extremity of its arms, and 8. bvpunctatus in 

 the character of the spines on the ventro-lateralia. 8. ocellatus and 8. argus are 

 specialised in the peculiar nature of their armature. All these forms are spinous, 

 their specialisation lying in other directions. 



Genus — Hadrandeeaster. 



The majority of the plates on this form are spinous, although a few distal 

 plates are smooth, probably indicating a catagenetic tendency in this direction. 



