122 



FOSSIL ASTEROIDEA. 



I have also met with C. Smithise and P. punctatus from this horizon. They 

 are readily distinguished by the characters which have already been given in 

 the key-table. 



NOTES ON THE KEY-TABLE. 



Genera — Calliderma, Nymphastee. Pentagonaster. 



The following are the chief distinctive characters which separate these three 



genera in recent forms. 



Calliderma. 



1. Arms well produced. 



2. Abactinal area covered with closely 



fitting plates. 



3. Ventro-Iateral and infero-marginal 



plates with prominent spines. 



4. Armature of the adambulacral 



plates consisting of 14-16 small 

 spines arranged uniserially, 

 with three or four rows of 

 larger spines arranged rather 

 irregularly. 



Pentagonaster. 

 Arms slightly produced. 

 As in Calliderma. 



Ventro - lateral and infero - 

 marginal plates without 

 prominent spines. 



Armature of the adambu- 

 lacral plates arranged in 

 longitudinal series. Series 

 on the whole uniform in 

 character. 



Nymphaster. 



As in Calliderma. 



Abactinal area covered with 

 paxillae, which are not 

 closely fitting in the radial 

 areas. 



As in Pentagonaster. 



As in Pentagonaster. 



The fossil species of Calliderma possess the characters numbered 1 and 2, but 

 differ to a greater or less extent in characters 3 and 4, in which they resemble 

 Pentagonaster. The genus Calliderma was, however, founded by Gray on one 

 species — C. emma. No other recent species has been assigned to the genus. It is 

 difficult, therefore, to say how far the distinctive characters of the Cretaceous 

 genera should have generic value. The question is debated by Mr. Sladen on p. 5 

 of this Monograph, and the very striking general resemblance of the fossil species 

 to the recent G. emma, influenced him in his decision to group them under this 

 genus. There appears no great reason to dispute this assignment, but I am in 

 more doubt as to the systematic position of the species which have been placed in 

 the genera Pentagonaster and Nymphaster. It will be seen that as the fossil 

 species of Calliderma resemble Pentagonaster in characters 3 and 4, the only 

 distinctive character which remains between the two genera is the length of the 



