A MONOGRAPH OF THE EXISTING CRINOIDS 209 



(usually 15-23) segments most of which are usually about as long as broad bearing 

 dorsally a transverse ridge; and P 2 is enlarged, stiffened, and more or less strongly pris- 

 matic, with the distal portion of the prismatic ridges on each segment produced out- 

 ward into a more or less broad finlike process or broad spine, a long spine with a broad 

 base, or a tuft of spines; P a is usually present, but may be absent. 



Geographical range.- From Fuchow, Province of Fukien, China, and the Philip- 

 pine Islands southward to the Tonga Islands, New Caledonia, Port Curtis, Queens- 

 land, and Baudin Island, Western Australia, and westward to the east coast of Africa 

 from the Red Sea southward to Bagamoyo, Tanganyika Territory. 



Bathymetrical range From the shore line down to 91 (?183) meters. 



Remarks. The genus Oligometra includes two species, one of which (carpenteri), 

 confined to the north Australian region, is remarkably constant in its characters, while 

 the other (serripinna) , ranging from eastern Asia to east Africa, both individually and 

 geographically is exceedingly variable indeed one of the most perplexing of all 

 comatulids. 



The genus Oligometra is closely allied to Decametra, from which it differs in its 

 characteristic P 2 . But in serripinna P 2 is very variable, in some forms approaching 

 the conditions found in some of the species of Decametra. 



The species of Oligometra are found most abundantly in the littoral and sublit- 

 toral regions. They are rather local, but the individuals are usually common in the 

 areas in which they occur. 



History. The genus Oligometra was proposed by me in 1908, with the genotype 

 Antedon serripinna P. H. Carpenter, 1881. Included in this new genus were adeonae, 

 bidens, caribbea, japonica, pinniformis, serripinna, and carpenteri. The genus was 

 referred to the family Himerometridae. In 1909 another list of the species of Oligo- 

 metra was published, including those given and in addition gracilicirra, imbricata, 

 and pulchella, subsequently described. Later in 1909 Oligometra, Cyllometra, Colo- 

 bometra, and Cenometra were associated in a new family, Colobometridae. 



Since 1909 Oligometra has been progressively restricted in scope by the removal 

 to other genera of most of the species originally included ; pinniformis was removed to 

 Amphimetra in 1911; Analcidometra was created for caribbea in 1911, Oligometrides 

 for adeonae (of which bidens is a synonym) in 1913; and Cotylomelra for gracilicirra 

 in 1916; japonica is herein referred to Iconomelra (see page 94). 



This leaves in Oligometra only carpenteri and serripinna, imbricata and pulchella 

 being forms of the latter. 



KEY TO THE SPECIES IN THE GENUS OLIGOMETRA 



a 1 . Segments of Pz, except for one or two of the minute terminal ones, much broader than long with 

 the edge toward the arm tip produced into a high uniform keel, and a high prominent tubercle 

 or short blunt spine in the distal half on the edge toward the arm base (from the Aru Islands 

 south to Port Curtis, Queensland, and Baudin Island, Western Australia; 15-57 me- 

 ters) carpenteri (p. 210) 



a 2 . Segments of PI mostly, or largely, longer than, or at least as long as, broad, with the distal ends 

 of the prismatic ridges more or less strongly produced, or with one or more spines (Province 

 of Fukien, China, the Macclesfield Bank, and the Philippine Islands, southward to the Tonga 

 Islands, New Caledonia, and New Guinea, and westward to the east coast of Africa from the Red 

 Sea southward to Bagamoyo, Tanganyika Territory; 0-91 [?183] meters) - - - serripinna(p. 216) 



