184 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



daris affinis. Indeed it is possible that some of the specimens of C. tribuloides 

 with long, tapering spines, which have been collected in the West Indies, are 

 really hybrids between that species and affinis, bnt there is no proof that this is 

 the case. There are only 3 valid recent species of Cidaris, and they are quite 

 sharply distinct from each other. The form which Doderlein ('87) described 

 under the name galapagensis is not constantly distinct from thouarsii and must be 

 referred to that species. All of the living species are littoral forms, and rarely 

 occur at a greater depth than 50 fths., but are found along nearly all tropical 

 and subtropical coasts. Numerous fossil species from Tertiary, Cretaceous, 

 Jurassic, Triassic, and even Permian strata have been named. The following key 

 to the recent species is based on the examination of 815 specimens representing 

 all three. 



Key to the Species. 



Small, h. d. rarely exceeding 30 mm.; median areas .45-60 of ambulacral 

 width, usually bare and often sunken ; abactinal system .45-.50 h. d. ; 

 genital plates always clearly in contact with each other; coronal 



plates 5 or 6, rarely 7 metularia 



Medium to large, h. d. 30-70 mm.; median areas seldom more than .40 of 

 ambulacral width, always provided with miliary tubercles ; abactinal 

 system usually less than .45 h. d. ; some or all of genital plates sepa- 

 rated in mature specimens ; coronal plates 7-10, rarely 11. 



Median interambulacral area more than .10 h. d. ; abactinal system 

 usually over .40 h. d. ; small spines olive, fawn-color, or red- 

 brown, with tips usually darker tribuloides 



Median interambulacral area less than .10 h. d. ; abactinal system 

 usually less than .40 b. d. ; small spines dark red-brown, purple, 

 or nearly black, with tips not noticeably darker thouarsii 



Cidaris metularia. 



Cidarites metularia Lamarck, 1816, Anim. s. Vert., 3, p. 56. 



Cidaris metularia Blainville, 1830, Zoophytes : Diet. Sci. Nat., 9, p. 212. 



Plate Ig, fig. 1, Rev. Ech., A. Agassiz, 1873. 



Although having a far more extensive range than either of the others, this species 

 shows much less diversity in the length and form of the primary spines ; they are 

 generally about .SO h. d. and are rarely if ever 1.20 h. d. The stalks of the large 

 globiferous pedicellariae have a well-developed " limb." The colors are generally 

 brighter than in the larger species, and the cross-bauding of the primaries is 

 usually very distinct; some Hawaiian specimens are very red, more or less 

 marked with yellowish or reddish white. The geographical range is from Cape 

 of Good Hope, northward on the east coast of Africa into the Red Sea (including 

 Madagascar, Mauritius, Bourbon, and the Seychelles), thence eastward along the 



