284 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 11 



4. Inner layer consisting of regular smooth buttons. Tables more 

 or less well developed, with disk and small teeth, few to many 

 on the tip of the spire 5 



4. Inner layer, if present, not as regular smooth buttons but as 

 rosettes, rods or plates, or partly irregular, more or less 

 twisted buttons. Tables variously developed, lacking in one 

 genus 6 



5. Buttons large, thin, with 3 to 5 pairs of small holes. Animals 

 flattened, with numerous ventral feet, large ventral tentacles, 

 and few warts on dorsum. Under rocks, {difficilis group) 

 2. Microthele Brandt, Panning emend. 



5. Buttons small, with three pairs of holes, large to small, even 

 to the point of obliteration. Cylindrical to flask-shaped forms, 

 with small terminal tentacles. Concealed among rocks or in 

 sand, {arenicola group) . . 3. Brandtothuria n. gen. 



6. Tables lacking except as vestiges in young individuals. Spi- 

 cules spinous to smooth rods, or small plates or curved bars 

 with a varying number of holes. Soft-skinned, with numerous 

 feet on the ventrum and small papillae on dorsum. Tentacles 

 large, often bushy, almost dendritic. Living attached to rocks, 



often in surf zone, {lubrica group) 



10. Selenkothuria n. gen. 



6. Tables present, though in some forms rare 7 



7. Tables of moderate size or small, with small disk. Spicules in 

 inner layer rosettes or small perforated plates or incomplete 

 buttons. Free living, in lagoons 8 



7. Tables large, delicate or robust, either alone or with mostly 

 irregular, twisted buttons 9 



8. Tables with small disk, moderate to high spire and 8 to 12 

 sharp spines on top, eight forming a Maltese cross. Inner layer 

 small rosettes or small perforated plates, {atra group) . . 

 9. Ludwigothuria n. gen. 



8. Tables low, degenerate, often incomplete. Inner layer as 

 smooth buttons, often incomplete, or as oblong rosettes. 

 {leucospilota group) . . 5. Mertensiothuria n. gen. 



9. Tables alone, numerous, with large disk and four slender rods 

 arising as parallel columns and turning outward at the top; 

 one basal crossbar. Smaller tables with reduced spire in the 

 feet. Slender, cylindrical to flask-shaped, with few feet in 



scattered rows, {maccullochi group) 



7. Irenothuria n. gen. 



