82 U-S. XATIOXAL MTSETM BULLETIN' 295 



Three subfossil specimens in material from Challenger station 24 at 

 Culebra Island. 



Subfossil specimens 581, from Albatross stations 2385, 2751, 2763. 

 2808, 4693, 4723, and 4728; and from Anton Bruun stations 360B. 

 361B. 363G, J. K, 365B. D. E. 366A. 369A. D. 370B. 373J. 397D. 

 399, 407, 409A. 



Dimensions.— Adult female U5XM 121353 from USXM Ace. No. 

 271766, left valve, length 1.68 mm. height 1.11 mm: right valve, 

 length 1.70 mm, height 0.99 mm. 



Left valve specimen USXM 121354 from Albatross 2808, length 

 1.33 mm, height 0.89 mm. 



Distribution'. — Described by Brady fl880j from the southeast 

 Pacific; possibly reported as Bairdia rictrix Brady by Brady (1880^ 

 from the West Indies. North Atlantic, Azores, off Brazil, Kerguelen 

 Islands, AustraUa, and/or Tristan d'Acunha. Identified in the material 

 under study from the southeast Pacific Ocean. Galapagos Archipelago. 

 Caribbean Sea, Mississippi Delta, east of Rio de Janeiro, and Mozam- 

 bique Channel; depth range 400-3475 meters. 



Affinities. — This extremely abundant and widespread population 

 probably includes more than one species and or subspecies, but I can 

 find no reUable criteria at present by which to establish consistent 

 subgroups. There is some variabihty apparent in the length and posi- 

 tion of the posterior caudate extension, as well as in the length and 

 slope of dorsomedian outhne of right valve and dorsal curvature of 

 left valve. 



The identification as Bairdia hirsuta Brady is based on the similarity 

 of the few and pooriy preserved specimens from Albatross locaUties in 

 the southeast Pacific to the lectotype specimen from this same region. 

 They are somewhat larger, more caudate, and show visible radial pore 

 canals. 



The southeast Pacific forms are quite similar in carapace shape to 

 those from the Gulf of Mexico and Caribbean: these, in turn, cannot 

 at present be distinguished from the rather variable population of the 

 Mozambique Channel collections. 



The Bairdoppilata identity is based on soft parts of the females 

 collected hving in the GuLf of Mexico, and on barely visible bairdop- 

 pilatan hinge dentition of subfossil specimens in this region. The 

 antennae and thoracic legs of these females are most exceptionally 

 long and thin but completely bairdoppilatan. Bairdoppilatan dentition 

 is not \isible on specimens from any other populations: possible ex- 

 planations include generally poor preservation, low penetrance of this 

 character, and misidentification of non-Bairdoppilata species -^-ith 

 convergent carapace shape. 



