136 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



fornia. After a careful comparison of numerous excellent specimens from Mexico, 

 Panama, and the Galapagos, it is clear that there is no constant character by which 

 C galapagenns Dbderlein (37) can be distinguished from thouarsii. Specimens 

 from the Galapagos usually have the short and very stout spines figured by D6- 

 derlein, and apparently do not have trideutate pedicellariae, but some Galapagos 

 specimens have long, slender, tapering spines, while some from the coast of Mexico 

 have spines like those of most Galapagos specimens ; and individuals from 

 Panama occasionally lack the trideutate pedicellariae. Doderleiu's present opinion 

 (1906) seems to be that galapagensis should be regarded as a variety of thouarsii. 



PHYLLACANTHUS. 



Phyll acanthus Brandt, 1835, Prodrome, p. 267. 



Test much as in Cidaris hut thinner ; thickness of an ambulacral plate only .30- 

 .40 of its horizontal length ; coronal plates vary greatly in different species, ranging 

 from 5 to 11 ; areolae not at all sunken and usually quite distinct even near actino- 

 stome; median interambulacral area not deeply sunken, though it may be bare and 

 FUtural lines distinct; ambulacra .20-.40 of interambulacra in width; poriferous 

 zones little sunken ; median ambulacral area generally with a double series of 

 marginal tubercles (inner much smaller than outer) and 1-4 additional, more or 

 less regular, vertical series between ; but when ambulacra are very narrow, median 

 area may he as in Cidaris, and when very broad, median area may be bare and 

 without additional tubercles; pores nearly or quite horizontal; distance between 

 two usually much greater than diameter of pore ; surface of interval fiat or hori- 

 zontally grooved, so that pores are connected by a furrow. Abactinal system much 

 as in Cidaris. Actinostome varies greatly in different species. Primary spines 

 exceedingly variable, usually 1.5-3 b. d. and quite stout ; actinal primaries either 

 as in Cidaris or somewhat flattened, thick and truncated at tip, slightly curved and 

 somewhat serrate. Secondary spines flat, but length and breadth very variable. 

 Large globiferous pedicellariae variable in form and often ^entirely lacking, but 

 tridentate and small globiferous pedicellariae are generally present. 



Large specimens of this genus are easily recognized, but small ones are often 

 puzzling. In very young specimens the pores are arranged much as in Cidaris, 

 and this condition has not wholly disappeared in specimens 20 mm. in diameter ; 

 in thomasii even the largest specimens do not have the interval between the pores 

 perfectly fiat. On the whole the genus is difficult to characterize properly and the 

 recent species are not well defined. But the combination of characters mentioned 

 in the key to genera is unlike that of any other cidaroid, and with proper care a 

 specimen of Phyllacanthus over 30 mm. h. d. should be recognized without great 

 difficulty. No other genus, however, shows so great diversity in the length and 

 form of the spines, and, as might be supposed, the pedicellariae are also very varia- 

 ble. There seem to be only five valid species in this genus, but it must be con- 

 fessed that the confusion of baculosa with annulifrra, and the latter with Stepha- 

 nonidaris bispinosa, has led to a most unfortunate situation, and there can be no 

 doubt that a careful revision of the genus based upon abundant material from the 



