130 THE VOYAGE OF H.M.S. CHALLENGER. 



19. Antedon denticulata, n. sp. (PL XXII. figs. 1, 2). 



Specific formula — A.y, 



Description of an Individual. — Centro-dorsal hemispherical, with a denticulate rim. 

 Twenty to twenty-five rather slender cirri, of twenty-five to thirty smooth joints, nearly 

 all of which are longer than wide, the fifth and sixth longest. 



First radials not visible ; second short and rather convex in the centre ; axillaries 

 short and widely pentagonal with slight backward projections. Both the radials and the 

 first two brachials are wall-sided, with straight edges and the margins of the dorsal 

 surface flattened. 



Ten arms ; the lower joints nearly oblong and the following ones smooth, short, and 

 bluntly wedge-shaped, gradually becoming more oblong about the middle of the arm. 

 Syzygies in the third and twelfth or thirteenth brachials ; others at intervals of four to 

 six joints. 



The pinnule on the second brachial is rather longer than that on the third, and the 

 length increases to the third pair (on sixth and seventh brachials). These consist of 

 about a dozen joints, the lowest of which are broad and slightly carinate. The next pair 

 are smaller with relatively longer joints and the following ones increase slowly in length. 



Pinnule-ambulacra not plated ; sacculi apparently absent. 



Colour in spirit, — very light brown. 



Spread, perhaps 14 cm. 



Locality.— Station 190, September 12, 1874; lat. 8° 56' N., long. 136° 5' E.; 

 49 fathoms ; green mud. One specimen. 



Remarks. — This species and the following one are readily distinguished from all 

 those previously described with wall-sided arm-bases by the entire absence of any 

 ambulacral skeleton on the pinnules ; but they differ altogether from one another, 

 especially in the characters of the pinnules and of the arm-joints. Antedon denticulata 

 has quite short arm-joints, the lower ones obliquely quadrate and their successors more 

 nearly oblong, but always much wider than long (PI. XXII. fig. 1), and the third pinnule 

 is the largest. But in Antedon pusilla the arm -joints are as long or longer than wide 

 (PL XXIII. fig. 1), and the first pinnule is the largest. 



There appear to be no sacculi in Antedon denticulata, or at any rate I have been unable 

 to find them. Their absence is remarkable in a form with unplated ambulacra, especially 

 as they are abundant in four individuals of Antedon fluctuans, which were dredged at 

 the same station. 



