54 



BULLETIN 76, UNITED STATES NATIONAL MUSEUM 



northern race, is notable for its slenderer rays and relatively smaller disk, char- 

 acteristics of the adults. Both have a complete series of adradial plates. 



Specimens with R between 40 and 50 mm. are very numerous, while those with 

 R over 50 are rare. These small examples are sexually mature; in fact, fairly well- 

 developed gonads were found in a specimen only 22 mm. major radius. 



These smaller specimens differ from the largest chiefly in having relatively 

 shorter rays, an incomplete third actinolateral series of plates, usually few adradial 

 spines (or sometimes none), fewer miliary spinelets (but not always), less extremely 

 flattened actinolateral spines, and two or three spinelets on the nonprominent adam- 

 bulacral plates. The ambulacral pores and feet are in four series and the pedicel- 

 lariae are generally fairly numerous and in size relatively as large as in the big 

 specimens. 



Type— Cat. No. 31640, U.S.N.M. 



Type-locality.— Station 5675, southwest of San Cristobal Bay, west coast of 

 Lower California; 284 fathoms; bottom temperature, 44.6° F. 



Distribution. — From Point Conception, Calif., to San Cristobal Bay, Lower 

 California, in 205 to 287 fathoms, green mud. 



Specimens examined.- — Seven hundred and fifty-one. 



Specimens of Myxoderma platyacanlhum examined 



Sta- 

 tion 



Locality 



Depth 



Nature of bottom 



Bottom 

 temper- 

 ature 



Num- 

 ber of 

 speci- 

 mens 



Collection 



2909 

 2960 

 3200 

 3201 

 '4321 

 4432 

 4433 

 4435 

 4436 



Santa Barbara Channel, 34° 22' N., 120° 08' 30" W. . 

 Santa Barbara Channel, 34° 10' 45" N., 120" 16' 45" W 

 Santa Barbara Channel 



.do- 



On" San Diego, Calif—. 

 Off Santa Rosa Island.. 



do 



Off San Miguel Island.. 

 do 



205 



267 



265 



280 



206 



272 



265-243 



287-274 



271-264 



Green mud. 



....do 



do 



do 



Dark green mud. 



Green mud 



do 



do 



do 



45.2 



48 



43.1 



42.9 



58 



No locality 



Off Point Conception. 



233 



Mud. 



118 



158 

 46 



141 

 2 



134 

 12 

 22 

 8 

 65 

 45 



U.S.N.M. 



Do. 



Do. 



Do. 

 Albatross, 1904 



Do. 



Do. 



Do. 



Do. 

 V. S. N. M. 



Do. 



i Adult specimens; all the others are either small or medium-sized examples. 

 1 Some specimens intermediate with rhomalcum. 



MYXODERMA PLATYACANTHUM RHOMALEUM Fisher 



Plate 14, Figures 3,3a; Plate 15, Figure 2; Plate 16, Figure 1; Plate 23, Figure 1; Plate 24, Figure 



2; Plate 25, Figure 3 



Myxoderma platyacanlhum rhomaleum Fisher, Ann. and Mag. 

 1919, p. 393. 



Nat. Hist., ser. 9, vol. 3, 



Diagnosis. — Differing from M. platyacanthum in having constantly broader and 

 slightly shorter rays, slightly larger disk, much heavier abactinal and marginal spines, 

 slightly shorter and decidedly heavier actinolateral spines, and shorter abactinal ped- 

 icellariae. R 100 mm.; r 12.5 mm.; R = 8 r; breadth of ray at base, 13 mm.; at a 

 little beyond base, 15.5 mm. Disk elevated, dome shaped; rays very gradually 

 tapered, but not attenuate distally. 



