ASTEHOIDEA OF NORTH PACIFIC AND ADJACENT WATERS — FISHER 15 



plate. If the dorsal integument of disk is stripped off so that the upper end of the 

 first two pairs of ambulacral plates can be seen, the length of the upper end of the 

 first pair will be seen to be about twice that of the second pair. In the following 

 species the length of the first pair is only slightly greater than that of the second. 

 In exilis the length of the third and succeeding pairs is about twice that of the second, 

 measured the same way. The articulation surface of distal face of second ambulacral 

 ossicle is narrowly oval, more or less acute below, the height about equal to width of 

 the combined pair. 



Actinostome 12.5 mm. in diameter. Mouth plates small, much narrower at 

 the inner than at the outer end, where the combined width is about equal to the 

 breadth of furrow at inner end of the plates. Armature: One short, delicate, acti- 

 nostomial spinelet directed across mouth of furrow, and one small, tapered, sharp 

 suboral spine at about the center of the plate. In the type about 20 per cent of the 

 mouth plates have a small spinelet at the corner adjacent to the first adambulacral, 

 and it is regularly present in a specimen from station 4387. This specimen has two 

 or three actinostomial spinelets to each plate. 



Gonads (testes) two to each ray, and opening on each side at a distance of 

 4 r from the base (opposite twentieth adambulacral). They have many slender 

 branched lobes. 



Type.— -Cat. No. 22348, U.S.N.M. 



Type-locality.— Station 4398, off San Diego, Calif., 32° 43' 20" N., 117° 42' 

 10" W., 620 fathoms, green mud, rocks; one specimen (disk and three rays). 



Distribution.— OB. southern California from San Diego to Santa Barbara Island, 

 448 fathoms to 1,059 fathoms, green mud. 



Specimens examined. — In addition to type: 



Station 4387 (probably 4387a is meant), vicinity of San Diego, Calif., 32° 32' 

 40" N., 118° 04' 20" W.; 1,050 fathoms, green mud; one specimen (disk and four 

 rays). 



Station 4416, off Sauta Barbara Island, Calif.; 448 fathoms, dark-green mud; 



one ray. 



Remarks.— I examined and compared directly with specimens of B. exilis a ray 

 of the type of B. tenella (Ludwig) from 1,322 fathoms, east of the Galapagos Islands. 

 Although the disk of tenella is not known, the presence of only two gonads to each 

 ray, as well as the general appearance of the rays, makes its generic position fairly 

 certain. B. tenella differs from exilis in having 40 or more very closely placed costae, 

 there being two, or sometimes three, to every adambulacral plate. The adambulacral 

 plates, in addition to the subambulacral spine, bear one or two adoral furrow spinelets 

 and one aboral furrow spinelet. The lateral spines, in spite of the crowded costae, 

 are less numerous than the adambulacral plates. They are at the upper border of 

 each of two consecutive plates ( . . ); then a plate is skipped (- ), somewhat as fol- 

 lows, the dashes representing the plates missed (sixth plate): ..-..-.-.- 

 .-..-..-..-..-, etc. 



The short swollen genital region and numerous crowded costae give to B. tenella 

 a considerably different appearance from that which is characteristic of the other 

 known species of this group. 



