ASTER OIDEA OF NORTH PACIFIC AND ADJACENT WATERS — FISHER 55 



Description. — The features which characterize this northern race are more readily 

 appreciated from a comparison of figures. The difference in the width of the ray, 

 for instance, is not great but it is constant even for small specimens and gives the 

 animal a stockier, more robust appearance. In the example of platyacanthum from 

 station 4321 having R 96 mm., a carinal spine at the base of the ray is a trifle over 

 2 mm. long and 0.5 mm. in diameter at the base. A similar spine in rJiomaleum 

 measures about the same length (1.75 to 2.2 mm.) and is 0.65 to 0.75 mm. in diam- 

 eter, and is a bit swollen and thick up to the blunt tip instead of being regularly 

 tapered and sharp. In the southern form the abactinal pedicellariae are 1 to 1.50 

 mm. in length and very appreciably longer than the miliary spinelets; in the north- 

 ern race they are shorter than or subequal to the spinelets and 0.6 to 0.8 mm. long. 

 The membranous investment of the spinelets is a little thicker than in the southern 

 form but it is rather too slight to be tangible. 



The specimens from off Pigeon Point, north of Monterey Bay, while less extreme 

 in the racial characters than the specimens from the type locality, belong to the 

 northern form. I think there can be scarcely a doubt that the two races intergrade 

 between Santa Barbara Channel and Monterey Bay. 



In large well-developed specimens the internal intermarginal ossicles are some- 

 times visible from the exterior. These are oblong ossicles which bind together adja- 

 cent transverse lobes of the supero and infero marginal plates. They are present 

 also for a short distance between the inferomarginals and first actinolateral series. 



Young. — As noted before, the difference in robustness can be distinguished in 

 small specimens. In an example of the southern form with R 15 mm., r is 2.5 mm., and 

 the breadth of the ray at base, 2. 75 mm. In a specimen of the northern form with 

 R 14 mm., r is 3 mm., or 20 per cent greater, and the breadth of the ray at base is 

 3.5 mm., or a little over 20 per cent greater, allowing for error in measurement. In 

 the case of specimens of the average size, say with R above 30 mm. and under 50 

 mm., the differences can be readily seen on comparison of specimens. An example 

 from station 2960 (southern form) has more delicate spines generally than are present 

 on an equal-sized example from 3112. (There are no small examples from Oregon.) 

 The difference in width of ray is slight, but is less in the southern example if R is 

 equal. 



Anatomical notes. — The stomach fills the disk and is single-chambered, there 

 being short lobes extending into the base of each ray. Through the dorsal surface 

 of these lobes empty two hepatic coeca which extend far along the ray. The intestinal 

 coecum is a trilobate, rather irregular sac, about 20 mm. long, proximally narrow, 

 distally expanded. The gonads form a multilobed mass opening on a level with the 

 inferomarginal plates 2 mm. from the interradial line. 



The prominent knob-like upper end of the first pair of ambulacra! plates is con- 

 nected with the interradial angle by a heavy calcareous buttress or bridge which is 

 really the very much enlarged first superambulacral ossicle. (PI. 14, figs. 3, 3a.) 

 Between the outer ends of these ossicles, which are compressed and rather thick dor- 

 soventrally, there extends centrally the narrow membranous interradial septum, with 

 a long dorsoventral free edge impinging upon the stomach. In the angle formed by 

 the superambulacral buttress (the apex of which is at the interbrachial line) there 

 is a slight depression bisected by the interradial septum, on either side of which there 



