Abstract.-Shells of 375 sexed specimens of Nautilus cf.  N. pompilius trapped at 300-700
ft (90-215 m) depth off Mutremdiu Point and Ngemelis Island, Palau, exhibit distinctive
dimorphism in shell proportion and overall size.  Of 221 mature animals, males have larger
shells (mean diameter = 209 mm), a broader aperture (mean maximum width = 99 mm;
mean width at apertural salient = 92 mm) and greater weight (mean shell + body weight
= 1426 gm).  Female shells are smaller (mean diameter = 198 mm), narrower (mean
maximum width = 91 mm; mean apertural salient width = 81 mm) and the overall weight
is less (mean = 1157 gm).  Dimorphic differences are not apparent in young shells (less
than 180 mm diameter), but develop during the final 1/2 to 1/4 whorl.  This is accompanied
by development of the spadix in males, indicating that shell dimorphism reflects sexual
maturity.  Of 375 animals trapped, 28,% were female; no depth segregation was apparent.
Traditional identification of larger fossil dimorphic cephalopods as females and smaller forms
as males is the reverse of observed dimorphism in Nautilus and should be discontinued in
 favor of macroconch and microconch designations.