66 HAWAIIAN AND OTHER PACIFIC ECHINI. 



Test with marginal slits, or lunules, or both. 



Not more than two marginal slits, and often none, in posterior half of test-margin. 

 Xo lunule in posterior interambulacrum. 



Two lunules or marginal slits present, one in each posterior ambulacrum Echinodiscus. 



Five lunules present, one in each ambulacrum Aslriclypeus. 



A lunule in posterior interambulacrum. 



Genital pores 5 Encope. 



Genital pores 4 Mdlita. 



Eight or more marginal slits in posterior half of test-margin Rotula. 



Echinarachnius. 



Gray, 1825. Ann. Phil., 26, p. 428. 

 Type, Sculella parma Lamarck, 1816. Anim. sans Vert., 3, p. 11. 



Although this genus is easily recognized, specific limits within it are very 

 perplexing and have been the source of some confusion and probably of no 

 little error with reference to the geographical distribution of the different forms. 

 The Northern Pacific Ocean, particularly the vicinity of Kamtchatka and Japan 

 seem to be the center of distribution for the genus. All three of the species 

 here recognized as valid occur there and two of them are not known from else- 

 where. The extraordinary range ordinarily attributed to E. parma requin 

 reinvestigation. The records from the Red Sea, Mauritius. Indian Ocean, and 

 Australia are almost certainly erroneous and there is no reason to believe that 

 the genus occurs south of 32° N. in either the Atlantic or Pacific Oceans. The 

 only characters which are available for distinguishing species are the shape of 

 the petals, the height and solidity of the test, the manner of branching of the 

 actinal ambulacral furrows, and the color. The last may be a very constant 

 and valuable character for all we know to the contrary but unfortunately the 

 red-brown pigment of clypeastroids (or at least of many of them) becomes 

 I ransformed into a beautiful green color on immersion in fresh water or alcohol, 

 and even undergoes more or less change in drying. Museum specimens of 

 Echinarachnius may thus show an extraordinary diversity of color, no shades 

 of which are necessarily distinctive. The use of color therefore as a specific mark 

 is open to serious objection, yet in sonic cases it is certainly helpful. The 

 shape of the petals and of the test are hardly Less uncertain characters than the 

 color and too much stress must not be laid on any one character. The forking 

 of the act inal ambulacral furrows is of course a much more fundamental character 

 than any of the others, and differences shown in this feature seem to be very 



