ASTEROIDEA OF NORTH PACIFIC AND ADJACENT WATERS — FtSHEB. 367 



just alike; and what is conclusive is the fact that young of a seven-rayed specimen 

 have sLx rays, while yount; of eif^ht-raycd specimens may have seven rays. 



The younj; {];row to a lart;e size before escapint;, by rupture of the supradorsal 

 membrane. The membrane first becomes thin over the younp, which are usuallv 

 found in the interradial region, but may occur also along the rays. The membrane 

 has the same appearance, before rupture, that the dorsal integument of Luulia 

 assumes before the autotomy of a ray. The fact that the young had its ventnd 

 surface close to the membrane, about to rupture, suggests that possibly its tiigestive 

 juices may play a part in the process of escape. After the esca])e of the young the 

 membrane closes, and the slit heals, causing probably tlie scarlike appearance of 

 the interradial smooth areas. 



This species stands quite alone, having no near relatives. Perrier, Verrill, 

 and Ludwig place it in the monotypic genus Ileiaster; Doderlein, Mortensen, and 

 Hamann, consider that it belongs in Pteraster. Perrier and Verrill indei>endently 

 made the species the type of a genus, so that it may be well to examine the original 

 descriptions. Omitting those characters of Perrier's diagnosis wliich apply equally 

 well to Pteraster, the following remain: Six rays, actinolateral spines iinmerseil in 

 the ventral membrane, not extcmling beyond the border of the ray. The second 

 character is nullified by Perrier's figure of the type. As a matter of fact, the lateral 

 fringe is present, though not well developed. The supradorsal membrane has 

 muscle fibers which are evident enough from the inner side; if these were not 

 present how would the membrane assume so many different forms or attain such 

 an unusual toughness and thickness ? Most of Perrier's diagnosis applies equally 

 well to Pteraster. 



Verrill (1S94, p. 277) says: "This species not only differs from other known 

 forms in having six rays, but appears to be peculiar in the presence of naked inter- 

 radial grooves and genital slits. This last character may be sufficient to warrant 

 its separation as a distinct subgenus (Temnastrr Verrill) or even as a genus. It 

 differs from our other species also in having fewer and stouter spines in the ventral 

 combs; in the broader and flatter ventral surface of the shorter rays; in the much 

 thicker skin of the ventral combs, and in the less evident comb of spines along the 

 margins of the rays. The dorsal membrane is also firmer and not at all granular; 

 the spinules over its surface are much more numerous, and the pores between them 

 are smaller and more numerous." 



The charact^-r of the adambulacral armature, the dimensions, the width of the 

 actinolateral membrane, and lateral fringe, the thickness of the supradorsal mem- 

 brane, the number of paxillar spines and spiracula are, as Verrill intended to show, 

 merely specific and not generic features. The generic characters mentioned in the 

 first sentence are: Six rays and int«rradial slits. The latter are temporary- open- 

 ings for the escape of the young, not permanent apertures. Furtliermore, as 

 probably holds true also for other species of Pteraster thesUts may occur elsewhere 

 in the supradorsal membrane. In small specimens there is not even a smooth 

 interradial groove. Wo are really thrown back upon the number of rays which is 

 not invariably six, but Is sometimes seven, eight, or nine. While it may be con- 

 venient to segregate Pteraster obscurus in a subgenus Ileiaster I do not think the 

 number of rays should constitut* a generic division, any more than in Anseropoda, 



