LAGANIDAE. 43 



ARACHNOIDIDAE (iieRoiy. 



This family is maintained for a single, monotypic genus, remarkablo for 

 the flatness of its test, the divergent petaloid areas, the supramarginal periproct, 

 the separate auricles and a plated buccal membrane. 



Arachnoides. 



Lcsku, 1778. Atld. ;ul Klriii, j). viii und p. 15-1. 

 Typp, Amchnniilea ecliinarachnius Leskr, /. c, = Echitiux placenta I,innc5, 1758. Sys. Nat., cd. 1(1, p. tiOij. 



(For a discussion of the nomenclatural questions involved in dating this 

 genus from Leske, see Clark, 1911, Ann. Alag. Nat. Hist., ser. 8, 7, p. 598). 



Arachnoides placenta. 



Echinus placenta Linne, 17.58. Sys. Nat., ed. 10, p. 61)6. 

 Arachnoides placsnta .\gassiz, 1841. Mon. Scut., p 91. 



Plate 125, figs. 1-3. 



Little need be said of this well-known species, but attention may well 

 be called to the fact that the pedicellariae which are very scarce and which 

 de Meijere calls tridentate, in reality have only two valves (PI. 125, fig. 2) ; these 

 valves measure .12-.25 mm. in length, are rather broad (PI. 125, fig. 1) and are 

 serrate and toothed at tip (PI. 125, fig. 3) where they meet. Many spines, 

 especially among the abactinal primaries, have asymmetrical swollen tips like 

 that figured by de Meijere (1904) for EchinocUscus. The geographical dis- 

 tribution of Arachnoides is remarkable for while it is common in New Zealand 

 as far south as Dunedin, and on the Australian coast, and reaches Samoa on the 

 east and the Malay Peninsula and Burmah on the north, it is not known from 

 either the western or the northeastern parts of the Indo-Pacific region. Very 

 few echinoderms are common to the Malay Peninsula and New Zealand, and no 

 other case is satisfactorily demonstrated. 



LAGANIDAE A. Agassiz. 



Although the superficial resemblance of the members of this family to Cly- 

 peaster is marked, more careful examination shows that the differences are far 

 more important and indicate that there is no very close relationship. The 



