REPORT ON THE ECHINOIDEA — MORTENSEN 



249 



The primary interambulacral tubercules are distinctly crenulate ; 

 the areoles are confluent except for the two or three upper ones and 

 are not deepened. The middle area is narrow, scarcely half as broad 

 as the areole. There is thus hardly room for any tubercules except 

 those of the scrobicular ring, which are small and little prominent- 

 The outer adradial part of the interambulacral plates is very narrow, 

 with no tubercules outside those of the scrobicular ring. There is a 

 narrow naked median line in the interambulacra which is slightly 

 depressed. 



The apical system (fig. 5) is rather elevated; the oculars come 

 near the periproct, which probably means that in adult specimens 

 they are insert. Genital openings have only just been formed. The 

 tubercules are few, and are con- 

 fined mainly to the middle of 

 the plates, as is usual in this 

 genus. It appears that the 

 interambulacral plates of the 

 peristome have just begun to 

 form. 



The primary spines are very 

 long, about four times the 

 horizontal diameter. (Prob- 

 ably the length will prove to be 

 relatively less in adult speci- 

 mens.) They are very slender, 

 scarcely tapering at all. Of 

 the isolated spines from station 

 5429 — which evidently belong 

 to this species — the longest is 116 mm., with a thickness of 2 mm. 

 They are very coarsely spinous through bearing spinules directed 

 obliquely forward which may show a more or less distinct arrange- 

 ment in three or four longitudinal series in the basal part, but other- 

 wise are without any serial arrangement. (PI. 52, fig. 2.) The sur- 

 face of the spines is otherwise very smooth and shining, with only 

 the faintest indication of a longitudinal striation when seen under 

 the microscope. The collar is short, only 2-3 mm. in length, thick- 

 ening only very inconsiderably downward. Some of the spines are 

 more or less widened at the tip; one of them has a small thickening 

 in the middle, due probably to some parasitic organism; the outer 

 layer is partially lacking here. One of the spines has a terminal por- 

 tion of nearly 20 mm. as yet undifferentiated. The oral primaries 

 (in the specimen in hand only the three proximal ones) are as usual 

 strongly serrate and curved; they are very slender. 



The secondary spines surrounding the primary spines are about 4 

 mm. long, flattened, very slender, and only very little broadened at 



Fig. 5.— Apical 



YSTEM OF HlSTOCIDAHIS ACUTISPINA, 

 NEW srECIES. X5 



