146 HAWAIIAN AND OTHER PACIFIC ECHINI. 



seems no reason to question the fundamental triradiate origin of even the 

 larger perforated plates, so that we have here again a suggestion of Diadematid 

 relationship. 



The Systematic Position of the Echinothurid^. 



In all discussion as to the proper position of the Echinothuridas in our 

 classification of the Echini, the crucial point is as to the stress that shall be 

 laid on the occurrence of regular series of ambulacral plates on the actino- 

 stome. If this character is regarded as of fundamental importance, then of 

 course the Echinothurids stand quite apart from all the other regular Echini, 

 excepting only the CidaridaB ; but if the possibility be admitted that this 

 character has appeared independently in the Eciiinothurids, or has been 

 retained by them, while lost by the other families, then we may well examine 

 with care the other characters of the family to see if we cannot ascertain its 

 relationships. 



Mortensen is one of those who hold to the fundamental importance of 

 the Echinothurid actinostome, and has expressed himself (1904) as favoring 

 their union with the Lepidocentridae in a suborder (Streptosomata) apart 

 from the other Ectobranchiate Regulares. Dciderlein, on the other hand, has 

 expressed himself (1906) as believing in the close relationship of the Echino- 

 thurids and Diadematids, and he recognizes a suborder (Diadematida) which 

 includes the Streptosomata as a special '' tribe" containing the Echinothurids 

 only and the Stereosomata, a tribe made up of the Diadematidos and its 

 closest allies. In attempting to reach a correct conclusion on the question, it 

 has seemed to us that it would be helpful to tabulate in parallel columns all 

 the morphological characters of the Echinothuridie and Diadematidae. By so 

 doing the similarities and differences will be made to stand out more vividly, 

 and we may perhaps be able to decide whether the resemblances indicate 

 relationship or not. The characters are arranged in what seems to us to be 

 the order of importance. 



The careful comparison of the ambulacra in Echinothrix, Astropyga, 

 Micropyga, Pliormosoma, Asthenosoma, and Sperosoma demonstrates that 

 the differences are superficial, and that in all these genera each ambulac- 

 ral plate consists of three elements, of which the middle one (primary) 

 is largest, while the adoral and aboral (secondary) elements show great 

 diversity in size and position. In Echinothrix the secondary elements are 

 rather large and lie next to the iutcrambulacrum ; the pore-pair of the 



