FISHERY BULLETIN: VOL. 86, NO. 2 



corner, the submarginal thickened zone and 

 groove are less distinct; the merus is not tilted 

 dorsally in normal position, and the palp is rela- 

 tively shorter and club shaped rather than curved 

 like a knife edge along the prehensile edge. The 

 lacinia of the first maxilliped is more angular 

 anterolaterally and has a smaller "portunid lobe". 

 The epistome is less lobulate than in B. thermy- 

 dron and the concave palatal area has much less 

 setose covering. The nearly toothless chelae have 

 brown fingers and there is no dense patch of setae 

 on the inner side of the palms. 



Comparisons with B. microps are necessarily 

 less complete because of the brief description of 

 the latter. The eyes are certainly not slender and 

 retracted in B. mesatlantica ; the chelipeds are not 

 noticeably dimorphic, and they are relatively 

 smooth rather than strongly granular and pilose 

 on the external surface as in B. microps. 



The distinctive exocular spots on the carapace 

 seem similar to those noted on the chelipeds of 

 Hypsophrys noar Williams (1974, 1976) and Mu- 

 nidopsis lentigo (Williams and Van Dover 1983). 

 Their function is unknown. 



Etymology. — From the Greek "mesos", middle 

 and "Atlantic", with reference to the Mid- 

 Atlantic Rift habitat. 



ACKNOWLEDGMENTS 



Contributors of specimens and crew members of 

 submersible vessels who helped to make the col- 

 lections are owed a special debt of gratitude for 

 securing the rare material described here. Donors 

 are acknowledged individually in each of the spe- 

 cies accounts. I am indebted to Keiko Hiratsuka 

 Moore for rendering the excellent illustrations, to 

 Ruth Gibbons for helping to produce the SEM 

 micrographs, and to F. A. Chace, Jr., R. B. Man- 

 ning, and B. B. Collette for critical review of the 

 manuscript. 



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