370 HAWAIIAN AND OTHER PACIFIC ECHINI. 



ECHINOMETRA. 



Gray, 1825. Ann. Phil., XXVI, (n. s. X), p. 426 (4 of reprint). 

 Type-species, Echinus lucunter Linn6, 1758. Syst. Nat., ed. 10, p. 665. 



Although this genus is so well characterized, one occasionally meets with a 

 specimen, usually young, in which the long axis is so little longer than the short 

 one, that it is necessary to measure the specimen to determine which is the long 

 axis. Such specimens have very naturally a close resemblance to individuals 

 of Heliocidaris and Strongylocentrotus, but they can usually be distinguished 

 without difficulty by the flatness of the test, which is generally very marked, 

 and by the large size of the auricles. 



The number of valid species of Echinometra is far from being satisfactorily 

 determined. There are few Echini so variable in form and color as E. Mathoei 

 and it may well be doubted whether ohlonga is anything more than an extreme 

 form of that species, and pida is probably not specifically distinct. Both these 

 species are retained however, that the points in which they differ from Mathcei may 

 be emphasized, in the hope that some zoologist who has a favorable opportunity, 

 such as the Hawaiian Islands afford, may make a careful study of the diversity 

 shown by the Indo-Pacific Echinometras. The American species, lucunter 

 and Van Brunti, are quite easily distinguished from the others by the large num- 

 ber of pore-pairs in each arc, but they are not so easily distinguished from each 

 other. The difference in the auricles however is surprisingh^ constant and will 

 separate all but very young specimens. The differences in the shape of the 

 test, the length of the primary spines, and the character of the trident ate pedi- 

 ccllariic seem to be less constant, though they are all useful. EUipsechinus 

 macroslomus Ltk. seems to be only a large specimen of Echinon^etra lucunter from 

 the eastern Atlantic Ocean. Large specimens from the Cape Verde Islands have 

 8 pore-pairs in many of the arcs and answer well to Liit ken's description, except 

 for (he position of tlie long axis. Dr. Mortensen has however reexamined 

 Lilt ken's type and has kindly assured me that the long axis is erroneously given 

 in the description, and that it really coincides with that of Echinometra. The 

 specimens brought home by the "Albatross" from Easter Island have proven 

 very interesting, for their superficial appearance is so much like that of Van Brunli, 

 if was not until the ambulacra were examined that their nearer relationship to 

 Mnlha'i became evident. The difference from Mntha") is however so constant as 

 to require a new species, insulnn's, for this interesting form. Jack.son (1011, 

 Mem. Boston 8oc. Nat. Hist., VII, p. 147) ha^s shown that lucunter and Van 



