122 



FOSSIL ASTEROIDEA. 



I have also met with G. Smithias and P. j^undatus from this horizon. They 

 are readily distinguished by the characters ^vhich have already been given in 

 the key-table. 



NOTES ON THE KEY-TABLE. 



Genera — Callidekma, Nymphaster, Pentagonastee. 



The follc^ving are the chief distinctive characters which separate these three 

 genera in recent forms. 



Callideema. 



1. Anns well produced. 



2. Abactinal area covered with closely 



fitting plates. 



Pentagonaster. 

 Arms slightly produced. 

 As in CaUiderma. 



Ntmphasteb. 

 As in CaUiderma. 

 Abactinal area covered with 

 paxilloe, which are not 



closely fitting in the radial 

 areas. 



3. Ventro-lateral and infero-marginal 



plates with prominent spines. 



4. Armatui-e of the adambulacral 



plates consisting of 14-16 small 

 spines arranged uniserially, 

 with three or four rows of 

 larger spines arranged rather 

 irregularly. 



Ventro - lateral aud iufero - 

 marginal plates without 

 prominent spines. 



Armature of the adambu- 

 lacral plates arranged in 

 longitudinal series. Series 

 on the whole uniform in 

 character. 



As in Pentagonaster. 



As in Pentagonaster. 



The fossil species of CaUiderma 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 CaUiderma 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 Ni/mjdiaster. It will be seen that as the fossil 

 species of CaUiderma resemble Pentagonaster in characters 3 and 4, the onlj- 

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



