480 BULLETIN 82, UNITED STATES NATIONAL MUSEUM 



are usually entirely visible, but they may be partly, or even almost completely, 

 concealed by the centrodorsal. They are short and are usually entirely or partially 

 united, though sometimes entirely free, laterally. The IBr 2 (axillaries) are short and 

 broadly pentagonal; rarely they are partially concealed by the centrodorsal. The 

 IIBr series are usually all present, and usually the outer IIIBr series on each postradial 

 series are developed, giving six arms to each postradial series arranged in 2, 1, 1, 2 

 order; but not infrequently all the IIIBr series are developed and one or more IVBr 

 series may occur, so that there are eight or more arms on some, most, or all the post- 

 radial series. The axillaries are often somewhat asymmetrical. The postradial 

 series are usually in fairly close lateral contact. They may be in very close apposition, 

 with produced and thickened margins and very strongly flattened against each other, 

 or they may be only hi partial contact or even slightly separated from each other. 

 The outer border of the postradial series is occasionally crenate. The division series 

 and arm bases are usually rather strongly convex dorsally. More or less developed, 

 though always low and broad, synarthrial and articular tubercles are usually present, 

 giving the proximal portion of the animal a characteristically rugose appearance; 

 but not infrequently these are obsolete or even entirely absent. 



The arms are 23-51 (usually 30-40) in number, 40 to 125 (usually between 55 

 and 80) mm. long. They may be short and composed of only about 100 brachials 

 or elongated with about 170 brachials. Usually they are of moderate length and 

 consist of about 150 brachials. The dorsal surface of the arms is usually more or 

 less rugose basally but otherwise smooth and is often smooth throughout. The arms 

 may taper gradually from the base to the tip, or they may increase slightly in width 

 up to about the fourteenth brachial, thence tapering distally. The first brachials 

 are wedge-shaped or almost rhomboidal, slightly longer externally than internally, 

 and closely united internally. The second brachials are shorter than the first and 

 are nearly oblong, though slightly longer externally than internally. The first 

 syzygial pair (composed of brachials 3+4) is commonly about as long as broad, though 

 often broader than long. The next four to six brachials are short and nearly oblong, 

 and those following are triangular or very obliquely wedge-shaped, nearly or quite 

 twice as broad as long, later becoming more bluntly wedge-shaped, and terminally 

 oblong, remaining broader than long until near the arm tips. Low and more or less 

 obscure articular tubercles are usually, though not always present on the arm bases. 



Syzygies occur between brachials 3+4, again from between brachials 13 + 14 

 to between brachials 31+32 (most commonly somewhere between the fifteenth and 

 twentieth brachials), and distally at intervals of 6 to 20 (usually 8 to 13) muscular 

 articulations. 



The lower pinnules may be approximately equal on all the arms, but usually 

 those on the outer side of the outermost arms of each postradial series, especially 

 P 2 , are more or less, and often greatly, larger than the others. Frequently the lower 

 pinnules on the outer side of the outermost arms arising from each IIBr axillary are 

 enlarged, those on the outermost side of the postradial series bearing the two IIBr 

 series being more enlarged than those adjoining the midradial line. The relative 

 amount of enlargement of the lower pinnules on the outer arms arising from a IBr 

 or IIBr axillary may vary greatly even in specimens of the same size from a single 

 locality. Sometimes the lower pinnules on the outer arms arising from a IBr or IIBr 



