46 U.S. NATIONAL MUSEUM BULLETIN 291 



not membranous. Because of this the web makes the head appear 

 longer and the arms shorter than they actually are. The section of web 

 between the third and fourth arms continues out the fourth arm as 

 the broad lateral keel or "tentacular sheath," 



All arms have aboral swimmincr keels or membranes. The keel on 

 the dorsal arms occurs as a low, fleshy ridge along the distal third of 

 the arms. The dorso- and ventrolateral arms have relatively well- 

 developed keels along their distal two-thirds. The lateral keel of the 

 fourth arms has been described above. 



The most distinctive feature of B. hacidifera is the structure of the 

 protective membranes that occur on all arms. Protective membranes 

 in the usual sense are lacking along the proximal half of the arms 

 and are replaced by long, fleshy, finger-like or rodlike projections that 

 roughly resemble thick, blunt cirri (pis. 8 a, b; 10 b, d). A trace of 

 a low membrane occurs between the bases of the projections, or trabec- 

 ulae. The trabeculae are not strong muscular rods, but are soft and 

 fleshy, nearly semigelatinous, i.e., the same consistency as the normal 

 protective membranes of Bathyteuthis. The trabeculae attain their 

 maximum dimensions immediately distal to their origin at the bases 

 of the arms. The ventral trabeculae of the dorsal three pairs of arms 

 are longer and more robust than those along the dorsal edges. Distally 

 the trabeculae become shorter, broader, and more lobate which gives 

 a scalloped effect to the .protective membrane that is fully developed 

 on the distal quarter of the arms. The protective membranes are normal 

 toward the arm tips. The ventral arms have fewer, smaller, less 

 strongly developed trabeculae that give way almost immediately to 

 scalloped protective membranes. 



The comblike rows of trabeculae appear early in ontogeny and are 

 present even in the smallest larva (6 mm ML) in the present sample. 

 In fact, this feature alone permits definite identification of the larvae 

 of this species. 



The oral surfaces of the arms are covered with numerous small to 

 minute suckers arranged in irregular rows. The arrangement of suckers 

 is biserial at the bases of the arms but quickly becomes irregularly 3- 

 to 4-rowed along the middle portion of the arm (pis. 8a,b; 10b,d). 

 The suckers become extremely numerous and closely packed in the 

 distal quarter of the arms, and toward the tips they become minute. 

 Suckers do not occur on the extreme distal tips of the arms. The 

 arrangement of suckers on the ventral arms is nearly the normal 

 biserial distribution. 



The tentacles are very long, thin, and muscular ; they are about 125- 

 150% of the mantle length. The tentacles are nearly round in cross 

 section except along the oral surface which is flattened and has a 



