A NEW GENUS OF SEA STARS — FISHER 449 



bulacrals behind mouth plates bemg at least partly separated. But 

 no species with numerous crowded rays can very well be constructed 

 on the plan of the asteriid actinostomial skeleton and at the same 

 time dispense with a strong adoral carina such as Plazaster possesses. 



Plazaster has a stout pillar of plates extending from the odonto- 

 phore to the dorsal wall, and involved in the partly membranous 

 interbrachial septum. Such a buttress might seem to be an adap- 

 tation required by the broader disk. Psalidaster and Labidiaster 

 have such a buttress, although Rathhunaster with a smaller disk and 

 deciduous rays has not. The ordinary small Pedicellasterinae do not 

 need a buttress to support the disk. The dorsoventral pillar is 

 characteristic of the Coscinasteriinae also, without relation to size of 

 disk; but in this group the ambulacrals become increasingly crowded 

 and the carinal and marginal systems of plates are strongly developed, 

 with voluminous stoles of pedicellariae on the spines. Nor is Plaz- 

 aster close to the Pycnopodiinae (Lysastrosoma, Pycnopodia), which 

 have very compressed ambulacrals like the Asteriinae. 



Possibly the best disposition of Plazaster is to regard it as a poly- 

 brachiate offshoot of the Pedicellasterinae, differing principally in 

 having a long adoral carina. 



PLAZASTER, new genus 



Rays 22 ^ to 35; skeleton reticulate with very numerous irregular 

 meshes extending to end of rays; plates small, usually with one small 

 acicular spinelet; superomarginals not differentiated or joined in 

 longiseries; inferomarginals small, monacanthid or diplacanthid, 

 alternating with a secondary spineless plate and joined directly to 

 adambulacrals ; no rudimentary actinals; adambulacrals diplacanthid 

 or triplacanthid, without spine pedicellariae; integument thin and 

 crossed pedicellariae attached in circles at base of spinlets; crossed 

 pedicellariae with jaw expanded distally bearmg two enlarged lateral 

 teeth; shank teeth numerous, small, in several longiseries; gonads 

 two to a ray, opening dorsolaterally at about diameter of disk distant 

 from base; a membranous interbrachial septum with strong dorso- 

 ventral pillar of plates from odon^ophore. 



Type, Labidiaster borealis Uchida. 



PLAZASTER BOREALIS (Uchida) 



Plates 66, 68, 69 



Labidiaster borealis Uchida, 1938, p. 800, fig. 9, pi. 33, figs 1, 2 (young). 



Description. — R, 180 mm., r 30 mm., R=6r, 35 rays. R 60 mm., 

 r 12 mm., R=5r, 33 rays. The skeleton is an irregular net of very 

 numerous small plates, usually 3-lobed at the nodes, elsewhere irregu- 



> Specimen with R 30 mm., r 5.5 mm. (Uchida). 



