72 HAWAIIAN AND OTHER PACIFIC ECHINI. 



sional specimens. The valves show great diversity in form, but are convex 

 or rounded at the tip ; they may be greatly constricted near the middle 

 (PI. 48, fig. 6) or not at all so (PI. 48, fig. 5). They are .40-.60 mm. in 

 length, and the stalk is 3-6 times as long. The heads of the stalks (PI. 47, 

 fig. 18) nearly always show a more or less evident constriction which is quite 

 characteristic. 



The triphyllous pedicellarifB are very scarce and small, the valves (PI. 48, 

 fig. 4.) measuring only .20-. 30 ram. in length. 



The pedicels, at least those of the actinal surface, in addition to the ter- 

 minal rosettes and supporting rods (PI. 48, figs. 7, 8) are provided with very 

 characteristic straight rods (PI. 48, fig. 9), which are expanded and more or 

 less perforated at the middle. The gills have large, more or less irregular, 

 knobbed and perforated plates or spheroidal masses of lime (PI. 47, fig. 19), 

 and similar but larger plates occur in the buccal membrane. These latter 

 are so large that they soiijetimes carry pedicellariaa. 



The sphoeridia are somewhat ellipsoidal, usually distinctly wider than 

 long. 



Tetrapygus. 



L. Agassiz and Desor, 1846. Cat. Eais., Ann. Sci. Nat. (3) VI, p. 354. 

 Type-species, Echinus niger Molina, 1782. Saggio St. Nat. Chili, p. 175. 



The peculiar structure of the ambulacra in this genus has been well 

 worked out and figured by Duncan and Sladen (1885), and appears to war- 

 rant its separation from Arbacia. Their attempt to attach Desmoulins' old 

 name (Echinocidaris) to it is, however, perfectly futile, for Echinocidaris is as 

 complete a synonym of Arbacia as could be found, and therefore, of course, 

 cannot be used in any other sense. Desmoulins himself recognized this fact, 

 but sought to maintain his name on the ground of priority. As a matter of 

 fact, Gray's name was published in April, and not in October as Desmoulins 

 asserts. Loven (1887) attempts to maintain Echinocidaris on the ground 

 that the first species mentioned by Desmoulins is m'(/er, although he calls it 

 pustulosa. This appears to be a pure assumption, however, and quite unwar- 

 ranted, so that the name Echinocidaris must be abandoned, notwithstanding 

 Duncan's (1891) redefinition of it. The name Tetrapygus is, however, avail- 

 able, for while the definition given by Agassiz and Desor is not based on the 

 structure of the ambulacra, the first species mentioned is niger, and that may 

 well be considered the type. 



