78 DISCOVERY REPORTS 



Fisher^ of Alaska is more nearly like P. hypernotius Sladen of the Antarctic than it is 

 like P. typiciis of the north Atlantic and adjacent Arctic. 



It seems to me probable that a Ctenodisciis species similar to procurator was formerly- 

 common to southern South America and Alaska, that later C. crispatus invaded Bering 

 Sea from the Arctic and extended its range southward, as it has done on the Atlantic 

 coast ; that there has been a commingling of two originally distinct forms in the north 

 Pacific, resulting in the highly unstable Ctenodisciis found there to-day. 



C. australis would be the south Atlantic equivalent of a relatively more stable north 

 Atlantic stock. It is worth noting that C. crispatus is a true high Arctic species which 

 extends into boreal waters, whereas neither C. australis nor C . procurator has been taken 

 in Antarctic waters. 



The largest specimens of australis have R 26 mm., r 14 mm., and 13 marginal plates 

 in each series. The largest north Pacific example of crispatus has R 40 mm. and 17-18 

 marginal plates ; but specimens nearly as large (R 29-35 mm.) have only 1 1-13 marginals. 

 Young. At St. WS 212 a series of 9 young was taken. The 5 smallest, 1-7 mm. in 

 diameter, are pentagonal, with relatively enormous terminal plates which occupy most 

 of the periphery. Each plate carries a dorsal spike-like sharp spine, nearly as long as the 

 minor radius, and 2 or 3 shorter terminal ones. There are 2 short marginal pairs of plates 

 and 3 rather rudimentary cribriform organs. The median is interradial and the others 

 are between the border of terminal plate and each marginal plate. These marginals 

 encroach upon the paxillar area about | r ; and centre of disk is raised into a low cone. The 

 close-set rudimentary abactinal plates have 1-3 tiny spinelets. There are 3 pairs of 

 relatively large tube feet on which the tip is no better developed than in the adult. Each 

 mouth angle has a prominent pair of plates with 4 or 5 short spinelets on the furrow 

 border (sometimes i occupies apex of mouth angle), and i prominent suboral on outer 

 part of plate. Adambulacral plates 2, the inner with 2, the outer with i spinelet. 



The largest specimen of the series has R 2-3 mm. and r 17 mm., a stellate contour, 

 and 5 cribriform organs on each side (2 supero- and 2 inferomarginals to each ray). The 

 largest paxillae have 4 or 5 short spinelets, while the madreporite is covered with 14 

 spinelets and looks like a large paxilla. Tube feet, 5 pairs ; adambulacrals 4, the first with 

 4, the others with 3 short sharp spinelets ; 2 actinal interradial plates on each interradius. 

 Food. Ctenodisciis evidently lives nearly covered by the soft mud or muddy sand of 

 its characteristic habitat, the conical elevation of centre of disk rising above the surface 

 as in the case of Astropecten. The cribriform channels of the marginal and actinal 

 surfaces are contrivances for maintenance of currents of water to and from the actinal 

 surface by means of the ectodermal cilia. 



If food is conveyed to the mouth by these cilia, it is not the principal source, since 

 mud is ingested in large quantities along with whatever contained objects are small 

 enough to pass through the mouth. Sometimes the abactinal surface is convex from an 

 overstuffed stomach, as in the case of a specimen from St. WS 805. In the fine sand and 

 muddy debris, in which simple sponge spicules are numerous, are 3 pebbles, the largest 



1 Fisher, 192S, pp. 35, 36. 



