A MONOGRAPH OF THE EXISTING CRINOIDS 171 



17, No. 2, 1938, p. 4. — H. L. Clark, Echinoderm fauna of Australia, 1946, p. 48 (in key), 



p. 52 (notes). 



Prometra A. H. Clark, Rec. Indian Mus., vol. 7, pt. 3, No. 2G, 1912, p. 267 (.nomen nudum), p. 269 

 (used in the combination Prometra [Decametra] brevicirra, nomen nudum); Ann. Mag. Nat. 

 Hist., ser. 8, vol. 10, No. 55, 1912, pp. 37-29 (used without comment as a generic name in the 

 description of Prometra laevipinna, P. minima, and P. parva); Crinoids of the Indian Ocean, 

 1912, p. 321 (diagnosis; genotype Colobometra chadwicki A. H. Clark, 1911; range); Unstalked 

 crinoids of the Siboga-Exped., 1918, p. viii (discovery of intermediates between this genus and 

 Decametra), p. 112 (in key, p. 125 (key to the incUided species).— Gisl^n, Vid. Medd. Dansk 

 Naturh. Foren., vol. 83, 1927, p. 2; Kungl. Fysiogr. Sallsk. Hand]., new ser., vol. 45, No. 11 

 1934, p. 18. 



Prometra {Decametra) A. H. Clark, Rec. Indian Mus., vol. 7, pt. 3, No. 26, 1912, p. 269. 



Colobometra (Prometra) (part) A. H. Clark, Crinoids of the Indian Ocean, 1912, pp. 321, 322. 



Amphimetra (part) A. H. Clark, Proc. Biol. Soc. Washington, vol. 26, 1913, p. 179. 



Diagnosis. — A genus of Colobometridae including small or medium-sized species 

 with 10 arms in which P, is shorter, more slender, and more delicate than Pj and is 

 composed of shorter segments, which have smooth or spinous distal ends; P, is absent; 

 and the proximal cirrus segments bear dorsally a transverse ridge that distally becomes 

 a pair of tubercles or small spines, or rarely a single median spine. 



Geographical range. — Southern Japan from Sagami Bay to the Korean Straits, 

 Hong Kong, southward to off the Clarence River, New South Wales, and Dirk Hartog 

 Island, Western Australia, and westward to the eastern coast of Africa from Suez Bay 

 south to Bagamoyo, Tanganyika Territory. 



Bafhymefrical range.- — From the shore line down to 137 meters. Of the 16 

 species, 11 have been taken in shore collecting, and 5 are known only as httoral forms. 

 The 6 species not as yet reported from the shore line have an upper limit of 9, 13, 18, 

 33, 55, and 64 meters. Nearly all the species are confined to shallow water; 7 are 

 found between 36 and 40 meters; 6 are found between 41 and 45 meters; 5 are found 

 between 46 and 55 meters; 4 occur between 56 and 70 meters; 3 are known from 

 between 71 and 90 meters; and only a single one extends deeper than 90 meters, 

 reaching a maximum depth of 137 meters, though occurring most frequently as a 

 littoral and sublittoral form. 



Remarks. — The genus Decametra as herein understood includes 16 species which, 

 taken as a whole, show a remarkable uniformity of structure. 



The cirri are IX to XXX, most commonly XIV-XX, in number, and are composed 

 of 10-29, most frequently 18-23, segments, which are subequal or increase shghtly 

 in length distally, and are from t\\nce as broad as long to about as long as broad, or 

 terminally slightly longer than broad. The earlier segments bear dorsally a distal 

 transverse ridge which soon moves to a median position and later usually, though not 

 always, becomes resolved into a pair of small blunted or pointed tubercles; rarely the 

 transverse ridge may persist as far as the antepenultimate segment, or it may become 

 narrowed into a single median spine or tubercle some distance pro.ximal to this. The 

 cirri vary from 2.5 to 14 mm. in length, being usually from 10 to 13 mm. long. 



The arms vary from 35 to 110 mm. in length, being usually between 35 and 70 mm. 

 long. The species with the longest arms are not necessarily the largest, for in some 

 forms the arms are, or may be, very slender and much elongated, while in others they 

 are stout and rather short. 



Pi varies from very small and weak, not more than half as long as Po, to nearly as 



