TRIPNEUSTES. 285 



TRIPNEUSTES. 



L. Agassiz, 1841. Int. Mon. Scut., p. 7. 



Type-species, Echinus ventricosus Lamarck, 1816. Anim. s. Vert., Ill, p. 44, = Cidaris esculenta Leske, 



1778. Add. ad Klein, p. XVII. 



As there seems to be practical unanimity in accepting Tripneustes, and as 

 there is little room for doubt that Hipponoe of Gray is technically a nomen 

 nudum, while it is also preoccupied, Agassiz's name for this genus should be 

 accepted. There can be little doubt that it represents the extreme development 

 of the Echinidae, the specialized abactinal system, deep gill-slits, wide ambulacra, 

 and great reduction of ambulacral primary tubercles all pointing to the same 

 conclusion. There are nominally three species in the genus -gratilla L. (long 

 known as variegatus) from the Indo-Pacific region, esculentus Leske from the 

 West Indies, and depressus A. Agassiz from the Western coast of Mexico. Re- 

 cent comparison of considerable numbers of specimens from various parts of 

 the world shows that the characters by which the species are supposed to be 

 distinguished are of slight importance and there is probably but a single species, 

 which is very variable in form, proportions, tuberculation, character of spines, 

 and color. It is true that all of the specimens with dark, slender spines are from 

 the Indo-Pacific region, but unfortunately the converse is not true for many 

 specimens from that region are as white and the spines are as coarse as in typical 

 esculentus. Owing to lack of sufficient material from the west coast of Mexico 

 the real status of depressus cannot be determined but it is doubtful whether it is 

 properly distinguishable from the West Indian species. The only character, 

 which has proved constant in the study of the material at hand, is the amount 

 of plating on the buccal membrane. With the aid of that feature, and the 

 comparison of tubercles and color, the three nominal species may be distin- 

 guished as follows: 



Buccal membrane with few, small scattered plates; tubercles rather large and numer- 

 ous, covering well the whole abactinal surface; spines white or yellowish .... esculentus. 

 Buccal membrane with numerous thick, moderately large plates; tubercles smaller 



and more irregularly scattered. 



Median interambulacral areas abactinally usually quite bare and dark colored; 

 tubercles usually very small; primary spines slender often very dark, but in 



some specimens, white gratilla. 



Median interambulacral areas abactinally not bare nor conspicuous; tubercles 



moderate; spines fairly stout, white depressus. 



