152 BULLETIN 82, UNITED STATES NATIONAL MUSEUM 



and arm bases are more or less thickly beset with small spines, at least along the borders 

 of the ossicles; and the arms are less than 15 (usually 10) in number, with the IIBr 

 series, when present, usually 4(3 + 4), sometimes 2. 



Geographical range. From the western Aleutian Islands, the Galapagos Islands 

 and Panama, and the Kermadec Islands westward to east Africa from Sokotra to the 

 Crozet Islands, and from Tristan da Cunha and Ascension Island northward to the 

 Canary Islands. 



Bathymetrical range. From 598 to 2,925 meters. 



Thermal range. From 1.2 to 7.88 C. 



Remarks. The genus Thalassometra is closely related to Oceanomftra, though on 

 the basis of our present meager knowledge it appears to be quite distinct. It is also 

 closely related to Leilametra, which differs from it only in the long straight tapering 

 cirri without dorsal spines or carination, recalling the relation between Craspedometra 

 and Heterometra, and in the large size of P 2 , which is intermediate in size and character 

 between PI and P 3 instead of being small like P 3 . The differences between Thalassometra 

 and Stiremetra are not very significant, though they appear to be constant. 



Nearly all the species of Thalassometra are known only from a few mutilated indi- 

 viduals from a single dredge haul, and in most cases the descriptions leave much to be 

 desired. 



The 16 species of the genus form, on the whole, a fairly homogeneous unit. The 

 species with numerous coarse spines or tubercles bispinosa, electrae, agassizii, and 

 villosa in which the IIBr series are 2 instead of the usual 4(3+4) may possibly repre- 

 sent a generic type distinct from those with very fine spines, but as yet we do not know 

 enough about them to enable us to form any very definite opinion. 



This genus and Stiremetra are the only representatives of the family Thalassometri- 

 dae at depths greater than 1,600 meters. 



History. As originally proposed in my first revision of the old genus Antedon 

 published in 1907, the genus Thalassometra included all the species known at that time 

 that are now included in the family Thalassometridae, with the exception of (Parametra) 

 compressa and orion, which were placed in Charitometra, but transferred to Thalasso- 

 metra in the following year. Since then Thalassometra has been progressively restricted 

 in scope by the establishment of Stylometra in 1908; Cosmiometra, Crotalometra, Para- 

 metra, Stenometra, and Stiremetra in 1909; Aglaometra in 1913; Daidalometra (separated 

 from Stenometra) in 1915; Oceanometra in 1916; and Horaeometra and Lissomctra (sepa- 

 rated from Parametra) in 1918. Leilametra was established in 1932, based upon a 

 newly discovered type, and Koehlermetra is separated from Crotalometra herein. 



KEY TO THE SPECIES IN THE GENUS THALASSOMETRA 



a 1 . Centrodorsal with the cirri arranged in 10 definite columns, two in each radial area. 



6>. Edges of the elements of the division series and lower brachials, and sometimes the dorsal surface 



also, with coarse blunt spines or tubercles, never finely spinous. 



c 1 . Elements of the division series and lower brachials bordered with numerous coarse blunt 

 spines; eighth and following brachials each with 2 or 3 long curved spines near the distal 

 edge; lower pinnules very spiny; 10 arms about 50 mm. long and the cirri about 17 mm. 

 long with 30-35 segments (between Marion Island and the Crozets; 2,925 meters). 



bispinosa (p. 154) 



