162 U.S. NATIONAL MUSEUM BULLETIN 234 



The arms are long (MAI 18-24), with the first arm always the 

 shortest and either the third or fourth longest (3.4.2.1, 2.4.3.1, 4.2.3.1). 

 The arms bear biserial suckers which are raised well above the arms 

 in some specimens, in others rather deeply set. The Sin is 2.5-4.0. 

 In the males, one or two of the suckers on the second and third arms 

 are especially enlarged with indices of 4.0-6.0. 



The third right arm is hectocotylized in the males and is somewhat 

 shorter than its fellows. The ligula is very small, with an index of 

 only 1.3-3.0. The sides are inroUed with faint traces of transverse 

 ridges. The calamus is minute. 



The web is shallow (WDI 10.5-20), with aU sectors nearly equal 

 although sector A is usually shallowest, the formula for the specimens 

 being C.B.D.E.A. for the female, B.C.A.D.E. and E.B.A.C.D. for 

 the two young males and D.B.E.A.C. for the older male. The web 

 extends along the ventral surface of the arms in niost cases. 



The penis is small, tubular, and slightly tapered, with a small 

 diverticulum at right angles to the remainder. The PLI is 19-20, 

 measured from the anterior end to the angle made by diverticulum. 



The radula was not examined. 



The sculpture consists of numerous small flat tubercles over the 

 dorsum of the mantle, head, and arms. Superimposed on these were 

 a number of large multifid cirri. There are three or four large and 

 several smaller branched cirri over each eye and several series of four 

 or five cirri on the dorsum of the mantle. Large branched warts 

 extend out along the aboral surface of the first two pairs of arms. 



The color in alcohol is a dark brownish black, slightly mottled on 

 the mantle and head. Along the outer surface of the arms there are 

 occasional lighter spots or blotches. The ventral surface of the 

 head and mantle bears numerous small reddish-brown chromatophores, 

 closely placed. The oral sm'face of the arms and web are a light 

 yellowish brown. 



The gills are large and stout, with six to nine filaments per demi- 

 branch. 



Measurements and indices of one female and three males of Octopus 

 horridus d'Orbigny, 1826, are: 



