DIADEMATID.E. 103 



The stout tridcntale are small, infrequent, and very characteristic. The 

 valves (PL 50, fig. 11) are only about .60 mm. long, but they are over .40 

 mm. broad and are noticeably deep. The blade is full of a calcareous net- 

 work and the tip is rounded or bluntly pointed. They are thus much more 

 like the ordinary form in A. tonswn than they are like those oi D. globulosimi. 



The ophicejjhalotts pedicellaria3 have the valves (PI. 50, fig. H) a trifle 

 larger and distinctly more pointed than in globtilosiim. 



The triphz/lloKS have the valves (PL 50, fig. 13) relatively shorter and 

 broader than those of glol)ulosum, and the cover-plate is not so deeply cleft. 



The spicules in the pedicels do not appear to have the ends drawn out 

 and imperforate as in glot)ulosum, but are narrow, irregular, perforated plates. 



The sphajridia are not peculiar. 



DIADEMATID.^ Peters. 



The Pedicellarle and Other Structural Characters. 



Plates 44, figs. 5, 6 ; 50, figs. 16-21 ; 51. 



The Diadematidfe show great diversity in the form of the pedicellarise 

 and the structure of these organs afi'ord useful characters for the distinctions 

 to be made between genera and species ; and yet no very great reliance can 

 be placed on them, because of the frequent absence of a characteristic form 

 and the intergradations shown by the same kind of pedicellarioe in closely 

 related species. As nearly all of the known species have been carefully 

 examined by Mortensen, who has described and figured the pedicellariae and 

 calcareous spicules in detail (1904), we have confined our figures to one or 

 two species not accessible to him and to the new species we have been called 

 on to describe. No less than seven different sorts of pedicellariie occur in 

 this family, two kinds of tridentate, three kinds of ophicephalous, one triphyl- 

 lous, and one globiferous. These are never all found in a single specimen, 

 and usually only three or four kinds occur. The globiferous pedicellariae are 

 found only in Centrostephanus, in which genus they are common and some- 

 times very abundant. In young individuals of the other genera only small 

 tridentate pedicellarite, usually with some triphyllous and ophicephalous, are 

 commonly found, the large tridentate appearing only with maturity. In 

 some specimens of Diadema and Chsetodiadema pedicellariae are infrequent 

 and the tridentate are very rare or wholly wanting. In Leptodiadema we 

 have failed to find any pedicellarise whatever. 



