1.20 COLT.T.CTIONS FROM MELANESIA. 



any one of the three collected by the officials of that great circum- 

 navigating expedition. 



Though the term Pleurechinus is due to L. Agassiz, and the specific 

 name bothryoides to his incomplete comprehension of the Cidaris 

 bothryoides of Klein and Leske, I have confined my "synonymy" 

 to Alexander Agassiz's Report on the ' Challenger ' specimens, as 

 it is there only that any such definite information is given as would 

 enable a zoologist to recognize examples of the species. The result 

 to which my own study of the specimens and of the definitions has 

 led me has, I think, been essentially confirmed by the investigation 

 into the structure of the test which, at my suggestion, Prof. Martin 

 Duncan was allowed to make *. 



Looking, first of all, at the general form of the test, the observer 

 is struck by its greater proportional height ; thus we find speci- 

 mens with an absolute diameter of 20, 18, or 17 millim. respec- 

 tively having a proportional height of 58 - 5, 66' 6, and 60. In 

 no known examples of any other species of Temnopleurus is the 

 proportional height more than 03*63 millim. f, ;md this is a rare 

 case, which obtains with a specimen only 11 millim. wide. The 

 examination of the dental apparatus did not shov. r any real point of 

 difference between this species and T. Tiardwickii. As in other 

 species of this genus, the abactinal area is much more prominent 

 in younger than in older specimens, while the characters of the 

 furrows between the plates is only intermediate between the ex- 

 treme condition presented by T. toreumaticus and that which is 

 seen in the species just mentioned. 



As to the minute construction of the test, Dr. Duncan finds that 

 " there is a generic relation between Temnopleurus and Pleurechinus, 

 and the only important distinction is the absence of crenulation in 

 the last-named type." To this crenulation or its absence Dr. Duncan 

 attaches more importance than do many naturalists who have devoted 

 themselves to this group, and he finds in consequence that " the 

 classificatory position assigned by A. Agassiz to Pleurechinus in the 

 ' Revision ' must be conceded, and it is a subgenus or section of 

 Temnopleurus T Notwithstanding the weight of the authority of 

 Agassiz and Duncan, I am bound to say that I feel still the im- 

 portance of the objections long ago urged by D'Archiac and HaimeJ, 

 " Quant aux crenelures des tubercules, on sait que cette particularite 

 n'a e'galement qu'une valeur tres-secondaire, puisqu'on la voit deja 

 disparaitre dans une certaine portion du genre Cidaris, sans qu'on 

 puisse decouvrir chez les especes a tubercules lisses aucune autre 

 difference concomitante; " and that being so, I can find no reason 

 which will justify the retention of a genus never very accurately 

 defined. 



The large t specimen in the ' Alert ' collection has a diameter 



"* Journ. Linn. Soc. (Zool.) xvi. p. 447. 

 t P. Z. S. 1880, p. 424. 



I Anim. foss. de PInde, p. 202 (18. r »3). The student should be reminded 

 that Prof. Martin (Notes Leyd. Mus. ii. p. 75) accepts the genus Pleurechinus. 



