34 U.S. NATIONAL MUSEUM BULLETIN 291 



ventral arms may be misleading in some specimens, because the sucker- 

 bearing portion may originate more distally than on the remaining 

 three pairs of arms, giving the impression that the fourth arms are 

 shorter. Generally, however, the fourth pair of arms is the longest or 

 equals in length the third and/or second arms. The first pair of arms 

 is always the shortest. 



The four pairs of arms are connected by a fleshy web (pi. 3a, b) 

 that is an extension of the integumentary layers of the head and arms. 

 The web that connects the first arms is the deepest, and web depth 

 decreases gradually between the firs:t and second, the second and third, 

 and the third and fourth arms. No web exists between the fourth 

 arms. Tlie web between the third and fourth arms is the continua- 

 tion of the lateral membrane (or "tentacular sheath") along the dor- 

 sal aboral angle of the fourth arms. This thick, fleshy web makes the 

 arms look shorter and the head (anterior to the eye opening) longer 

 than they actually are. 



The arms are elongate cones, thick at the bases and evenly tapered 

 to the tips (i.e., not sharply attenuate). Swimming keels and mem- 

 branes exist on the aboral surfaces of the arms, but they occur only 

 as fleshy ridges rather than as distinct, muscular (or membranous) 

 keels. The keel on the- dorsal arms is a raised fleshy ridge along the 

 distal half of the arm. The keel on the second arms is noticeable proxi- 

 mally as a thickened ridge that becomes raised and thinner distally. 

 The greatest development of a true swimming keel occurs on the third 

 arms w^here the keellike ridge is raised and relatively thin (though 

 still fleshy) along the distal three-fouriths of the arm. On the ventral 

 arms the lateral membranes, which partially envelop the tentacular 

 stalks during swnmming, are moderately developed, althougli they 

 too are fleshy, particularly proximally, and not membranous as in 

 most other oegopsids. 



All arms in B. dbyssicola are supplied with protective membranes 

 (pi. 3a) that are extremely variable; they are thick and fleshy. Typi- 

 cal thin, muscular, strutlike trabeculae are lacking. The membranes 

 appear to be the result of the fusion of the edges of flattened trabec- 

 ulae. The membranes arise abruptly on tlie oral surface of the first 

 three pairs of arms, about in line with the base of the proximalmogt 

 sucker. Along the proximal two-thirds to thi-ee-fourths of the first 

 three pairs of arms the protective membranes are well developed and 

 the edges Stand well alx)ve the level of the suckers. DiStally, how- 

 ever, the membranes become merely low ridges that form the oral 

 angles of the arms primarily ; suckers stand well above the borders of 

 the membranes. Borders of the protective membrane may be straight, 

 gently undulate, or scalloped to varying degrees. The protective mem- 



