A MONOGRAPH OF THE EXISTING CRENOIDS 115 



Syzygies occur between brachials 3 + 4, again from between brachials 16+17 to 

 between brachials 21+22, and distally at intervals of from 8 to 16 muscular articu- 

 lations. 



The lower pinnules are stout with broad carinate segments; they diminish in 

 size from the second to the seventh brachial and then increase slowly. The later 

 pinnules are styliform with the two basal segments slightly expanded. The basal 

 segments of PI have then* outer sides somewhat flattened, and the third-fifth have their 

 inner edges truncated so as to be flattened against the arm. 



The disk is about 10 mm. in diameter and is thickly covered with plates which 

 extend out on to the arms at the sides of the ambulacra and also over the gonads. 

 The pinnules have well-defined side- and covering-plates, most of the former being 

 notched for the presence of sacculi which are small, but pretty regularly distributed. 



The color in alcohol is dirty yellowish white. 



Notes. Carpenter said that this is a smaller species than A. valida; the cirri, 

 though containing the same number of segments, do not reach more than 50 mm. in 

 length as compared with 80 mm. in that species. The flattening of the outer side of 

 the basal pinnules is not so evident, and the lower segments of the distal pinnules 

 show but little trace of the expanded trapezoidal form that is so characteristic of 

 A. valida. The sacculi, too, are much more abundant than in the latter species, the 

 side plates being notched for their reception. The ambulacra extend onto the genital 

 pinnules, as is also the case in A. valida, but the plates covering the gonads are much 

 less developed than in the species like Poecilometra acoela, which have no ambulacra 

 on these pinnules. 



I examined the single specimen of A. incerta at the British Museum in 1910. 

 This is a large and robust species ; the edges of the ossicles of the IBr series are spinous ; 

 the well developed synarthrial tubercles resemble those of certain species of Psathyro- 

 metra. 



Prof. Torsten Gisle'n examined this specimen in 1925 and published notes on it 

 in 1928. He said that the synarthrial tubercles are carinate and the brachials are 

 bordered with spines. He suggests that it is perhaps a Thalassometra. 



Abnormality. In the single known specimen one of the postradial series lacked 

 the IBr], the IBr 2 (axillary) immediately following the radial. This axillary is smaller 

 and more triangular than the other axillaries so that the pair of first bracbials which 

 it bears are in close lateral contact with the axillaries of the postradial series on either side. 



Locality. Challenger station 170A; near the Kermadec Islands, north of New 

 Zealand (lat. 2945' S., long. 17811' W.) ; 1,152 meters; bottom temperature 4.17 

 C.; volcanic mud; July 14, 1874 [P. H. Carpenter, 1884, 1888; Bateson, 1894; Hartlaub, 

 1895;Hutton, 1904; A. H. Clark, 1907, 1909, 1912, 1913, 1918; Gisle'n, 1928] (1, B. M.). 



History. Antedon incerta was first mentioned by Dr. P. H. Carpenter in his report 

 upon the stalked crinoids of the Challenger expedition published in 1884. Carpenter 

 gave a specific formula for the species following Bell's method as modified by himself, 

 and noted that in this species, although the genital pinnules are protected by a very 

 close and regular pavement of anambulacral plates, the ambulacra extend over these 

 plates in the usual way. He noted that in Antedon incerta the protecting anambucral 



