CLARK: THE CIDARIDAE. 225 



and even this may be incomplete in small specimens ; between are more or fewer 

 scattered tubercles, but there is never a complete second series even in very large 

 specimens ; vertical sutural line, bare ; pores oblique, close together, surface of 

 interval rough or elevated. Abactinal system variable in size, oculars and especially 

 genitals with noticeably wide bare margins. Actinostome .30-. 45 h. d., with few or 

 no interambulacral plates. Primary spines, when fully developed, long, 1.5—4 h. d. 

 cylindrical or nearly so, white (sometimes tinged with rose, purple, or yellow) with 

 a darker collar; actinal primaries flat, somewhat curved, coarsely and sharply 

 serrate ; secondaries flat and not peculiar. No globiferous pedicellariae whatever ; 

 tridentate pedicellariae very variable in size (.30—6.0 mm.) and form, with 2-4 

 (generally 3) unusually stout, wide valves. 



This is one of the most distinct and easily recognized of the genera of recent 

 Echini, but the species it contains are most perplexing and are exceedingly diffi- 

 cult to distinguish from each other. The genus has a wide geographical range, as 

 it occurs in the North Atlantic Ocean, the Caribbean Sea, among the Galapagos 

 Islands, the Hawaiian Islands, the East Indian Islands, and the Nicobar Islands, 

 along the coast of Japan, near Australia, and along the east coast of Africa, it 

 depths ranging from 169 to 799 fths. Several species from the Tertiary have 

 been named, and serrate spines, like the actinal primaries of Porocidaris, occur in 

 the Jurassic. There is little diversity of color in the genus, for the test, the collar 

 of the primaries, and the small spines are commonly some shade of brown, often 

 becoming very dark or deep purple with age, while the primaries are usually very 

 white. The following key is based on the examination of 51 specimens representing 

 all the species, except misakiensis. 



Key to the Species. 



Pedicellariae all with 2 valves purpurata 



Pedicellariae mostly with 3 valves. 

 Abactinal system .40-.55 h. d. ; primaries rather stout (thickness of large 

 ones 3-6% of length), finely and sharply thorny. (These prickles 

 are not always easily seen with the unaided eye, but are so distinct 

 that a spine cannot be drawn upward between thumb and finger 

 when lightly closed upon it.) 

 Small spines in interambulacra, outside scrobicular circles, above am- 

 bitus, very few ; ambulacra almost wholly bare between marginal 

 rows of tubercles ; primaries stout, 1.5-2.5 h. d. (thickness 5-6 per 

 cent of length), often becoming larger and fluted near tip, with 



numerous (25-30) longitudinal series of prickles sharreri 



Small spines more numerous on upper half of test ; ambulacra usually 

 with scattered tubercles ; primaries somewhat less stout, with 

 about 12-15 longitudinal series of prickles, more or less tapering 

 and never enlarged and fluted at tip, but occasionally with large 

 projecting thorns near base. 

 Primaries less stout (thickness 3-4 per cent of length) ; no special 

 depression on inner surface of valves of large pedicellariae above 

 hypophysis ; test, secondaries, and collar of primaries light red- 

 dish- or yellowish-brown elegans 



vol. li. — no. 7 15 



