536 



THE VOYAGE OF H.M.S. CHALLENGER. 



Synopsis of the Genera included in the Family Echinasteeid m. 



A. Disk large. Rays numerous. Armed with large isolated spines covered 



with membrane beset with calcareous granules. Numerous madre- 



poriform bodies. Forficiform pedicellaria? present .... 



a. A single genus 



B. Disk small. Rays usually five. Armed with large spines beset with scales 



or asperities. One madreporiform body. No pedicellariae present. 



No interbrachial partitions 



a. A single genus .......... 



C. Disk small or medium-sized. Rays five or six. Spinulation small : spinelets 



isolate or grouped. No pedicellariae present ..... 



a. Armature of the adambulacral plates simple or disposed in transverse 



series. 



a. Abactinal plates bearing small spinelets, in more or less com- 



pact groups. 



o. Disk small. Marginal plates usually distinguishable. 

 Adambulacral plates with a small inner spinelet 



placed high in the furrow 



/?. Disk comparatively large. Marginal plates superficially 

 undistinguishable. No small inner spinelet on the 

 adambulacral plates ...... 



b. Abactinal plates bearing simple isolated spines 



b. Armature of the adambulacral plates disposed in a double longitudinal 



series. Abactinal plates forming a wide-meshed network 

 and bearing compact groups of spinelets .... 



D. Disk moderately developed. Rays five. Abactinal plates regularly dis- 



posed, bearing small isolated spinelets. Marginal plates with large 

 valvate pedicellarioe. Actinal intermediate plates bearing one or 

 more large flattened spinelets ....... 



a. A single genus 



AcANTHASTERIN.fi. 



. Acanthaster. 



Mithrodiin.e. 



. Mithrodia. 



ECHI NASTERIN.fi. 



Cribrella. 



. Perknaster. 

 . Echinaster. 



Plectaster. 



VALVASTERINfi. 



. Valvaster. 



Subfamily Acanthasterin^e, Sladen, 1888. 

 Genus Acanthaster, Gervais. 



Stellonia (pars), Agassiz, Mem. Soc. Sci. Nat. Neuchatel, 1835, t. i. p. 191. 



Echinaster, Gray, Ann. and Mag. Nat. Hist., 1840, vol. vi. p. 281. 



Acanthaster, Gervais, Diet. Sci. Nat., suppL, 1841, t. i. p. 474. 



Echinites, Midler and Troschel, Archiv f. Naturgesch., 1844, Jahrg. x., Ed. i. p. 180. 



Acanthaster is a very isolated type, having a fades altogether unlike that of any other 

 form. The genus is confined to tropical waters, and, though ranging over a wide area of 

 distribution, the amount of morphological plasticity exhibited is very slight. I feel some 

 uncertainty as to whether the form from Mauritius, so carefully described by de Loriol, 1 

 can really be recognised as a species independent from Acanthaster echinites; and lam 

 equally doubtful as to the validity of the claims of Acanthaster cllisii. 



1 Mem. Soc. Phys. et Hist. Nat. Geneve, 1885, t. xxix., No. 4, p. 6. 



