Figure 39. — Pcnaciis (M.) aztccus axtccus Ives. a. Rostrum, $ 35 mm. c.l. off Tampico, Mexico, 

 b. Rostrum, S 31 mm. c.l., off Tampico, Mexico. 



THORACIC APPENDAGES 



Third maxilliped reacliinor at least to base and, 

 at most, to distal end of second antennular seg- 

 ment; length of dactyl about two-thirds that of 

 propodus. First pereopod surpassing carpocerite 

 by one-third length of dactyl to seven-eighths of 

 propodus. Second pereopod exceeding carpocerite 

 by one-half length of dactyl to entire length of 

 propodus. Third pereopod reaching at least distal 

 end of antennular peduncle, and. at most, exceed- 

 ing it by as much as one-tenth length of carpus, 

 proportionately longer in larger individuals. 

 Fourth pereopod reaching at least distal end of car- 

 pocerite and, at most, exceeding it by length of dac- 

 tyl. Length of fifth pereopod subequal to that of 

 fourth pereopod. Exopods on all pereopods; long 

 ischial and basial spines on first pereopod ; rather 

 long basial spine on second pereopod. 

 ABDOMEN 



Carinate dorsally from posterior half of fourth 

 somite posteriorly, carina increasing in height pos- 



teriorly to form median keel on sixth somite, there 

 ending in sharp spine on posterior margin. Dorso- 

 lateral sulcus (figs. 42, 43) broad, ratio K/S rang- 

 ing from 0.50 to 3.50, modally 1.25 (fig. 44), 

 varying within same limits in males and females; 

 in small size classes to 29 mm. c.l. (fig. 45), modal 

 value of K/S higher than in larger individuals; 

 dorsal lip of sulcus rounded, ventral lip sharp. 

 Sixth abdominal somite with three conspicuous 

 cicatrices on each side, fii-st longest ; fifth abdomi- 

 nal somite with one cicatrix and series of minute 

 pits anterior to sinus on posterior margin of so- 

 mite, on rib in larger specimens. Fourth abdominal 

 somite with series of minute pits dorsal to sinus on 

 posterior margin of somite, also on rib in larger 

 specimens. Telson unarmed, with deep median 

 sulcus and sharp pointed tip. 

 PETASMA (fig. 46 a-d) 



Width of ventral costa increasing uniformly 

 proximally, mesiodi.stal portion fonning gentle 

 arc; distal portion unarmed along free border but 



532 



U.S. FISH AND WILDLIFE SERVICE 



