40 PEor. p. M. Duncan's eevision of the 



EcMnotliuriclse near tte Diadem atidge, granting some atavism, 

 than to station them at the end of the Palgeechinoidea ; the 

 alliance is with the first group, and the descent was probably not 

 through the line of Cidaridse and Saleniidse. 



The presence of rudimentary internal branchisB in Diadema, 

 according to Ludwig and the Sarasins, is another link with the 

 Diadematidae. It is proposed to place the Echiuothuridse in 

 a suborder of the Gruathostomes with external branchiae, the 

 Streptosomata. 



With regard to the Family Diadematidae and its 36 fairly- 

 defined groups, it may be said that the artificial method of classi- 

 fying with the aid of crenulation and non-crenulation of tubercles 

 gives way to the natural method in which the structure of the 

 ambulacral plates is of primary taxonomic importance. Six 

 former genera become subgenera, and the recognized genera are 

 grouped in 4 very useful Subfamilies (p. 59). 



I. Suborder Streptosomata (p. 25). 



Family Echinothtjeid^, Wy. Thomson, 1873, Depths of the 

 Sea, p. 164 ; 1874, Phil. Trans. Boy. Boc. vol. clxiv. pt. 2, 

 p. 730. A. Agassiz, 1881, Report on ' Challenger ' JEchini, 

 p. 71. Ludwig, 1880, Zeitschr. fur wiss. Zool. vol. xxxiv. 

 P. Si" F. Sarasin, 1888, Frgehn. Naturw. Forsch. auf Ceylon, 

 Bd. i. Heft 3, p. 129. 

 Moderate to very large, tumid or very depressed, thin, flexible 

 teats. The apical system with the plates usually separated. 

 Coronal plates with membranous edges, with imperfect develop- 

 ment of the calcareous element, which is highly and openly reti- 

 culate and usually thin. Ambulacral plates simple or compound, 

 with small plates intercalated in the membranous areas, and 

 tentacle -bearing, simple, low, perforated plates, within the peri- 

 stome, imbricating adorally. Tentacles triserial and heteropodous. 

 Interradial plates variable in shape, variably overlapping and 

 in the opposite direction to the ambulacral plates. External 

 branchiae small, internal very large. Spheridia may be present. 

 A series of internal longitudinal muscles. Jaws with a small 

 foramen without epiphyses, teeth grooved; perignathic girdle 

 continuous. Spines short, may be sheathed and capped or not 

 at the end, hollow and largely cellular within and with solid wedges 

 externally. Epistromal structures often highly developed. 



