of carpus. Third pereopod reaching at least, to 

 distal half of second antennular segment and at 

 most exceeding peduncle by entire length of dactyl. 

 Fourth pereopod reaching at least midlength of 

 carpocerite and at most exceeding it by entire 

 length of dactyl. Fifth pereopod equal in length to 

 fourth or surpassing it by 1/5 to 1^4 length of dactyl. 

 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 gradually increasing in 

 height posteriorly to form median keel on sixth 

 somite, ending in sharp spine on posterior margin. 

 Dorsolateral sulcus (fig. 52) usually narrow, ratio 

 of height of keel to width of sulcus from 

 2 to 8.5, modally 3 in the Cai"ibbean region and 

 eastern Brazil (fig. 53), and 3.5 fi-om the Gulf of 

 Paria to Camocim, Brazil (fig. 55) ; in the Carib- 

 bean region and eastern Brazil modal value of 

 K/S is 3 or less in all size classes (fig. 54) whereas 

 in the Gulf of Paria to Camocim modal values of 

 K/S range from 2 to 8 in the various size classes, 

 although figure 56 shows a strong tendency toward 

 a groui^ing of these values around 4 ; range of K/S 

 is same in males and females; dorsal lip of dor- 

 solateral sulcus rounded, ventral lip sharp. Sixth 

 abdominal somite with three prominent cicatrices 

 on each side, anterior one the longest; fifth ab- 

 dominal somite with one cicatrix and series of mi- 

 nute pits anterior to sinus on posterior margin of 

 somite; fourth abdominal somite with similar se- 



1 mm 



Figure 52. — Penaeus ( il.) aztecus subtilig Perez Farfante, 

 sixth abdominal somite, posterodorsal portion, $ 36 mm. 

 C.I.. Gulf of Venezuela. 



Figure .53. — Percentage of distribution of keel-sulcus 

 (K/S) values in Pcnaciis (J/.) aztecus subiilis P^rez 

 Farfante from the Caribbean region and eastern Brazil. 



ries of pits dorsal to sinus on posterior margin 

 of somite. Telson unanned, with deep median 

 sulcus and sharp pointed tip. 



PETASMA (fig. 57) 



Ventral costa increasing uniformly in width 

 proximally, mesiodistal portion forming gentle 

 arc, distal jiortion unarmed along free border — 

 rarely with two or three widely spaced spinules — 

 and with two to four series of small teeth arranged 

 in elongate, narrow patch on attached border; apex 

 of costa adnate to adjacent membranous portion of 

 ventrolateral lobule, latter with extensive armature 

 of thickly set spines. Distal fold rather small, 

 usually armed with spinules, often in large num- 

 bers, but occasionally plain. Distomedian jDrojec- 

 tions rather thick, and short, overhanging slightly 

 distal portion of ventral costae. 



APPENDIX MASCULINA 



Relatively elongate, length 1% to 1% times max- 

 imum width, armed with strong, short spines along 

 slightly concave outer margin and with longer 



WESTERN ATLANTIC SHRIMPS OF GENUS PENAEUS 



549 



