48 Annals of the Carnegie Museum. 



base of anal and anal margin black ; base of anal white ; dorsal and 

 adipose blackish. bilineatus. 



Order HAPLOMI. 



PCECILIIDsE. 



Genera of Pceciliid^e of Guiana. 



a. Males similar to the females except in color ; oviparous ; dorsal over the anal ; 

 eyes normal ; teeth conical, in several series ; body subcylindrical. 



Rivulus Poey. 

 aa. Anal of the male modified ; viviparous. 



b. Eyes elevated, the upper part adjusted to see in the air, the lower in the 

 water ; dorsal behind the anal in both sexes. Anableps Linnaeus. 



bb. Eyes normal. 



c. Caudal peduncle without a knife-like edge below ; anal fin of the male 

 on the lower surface of the abdomen. 

 d. Intromittent organ not spine-bearing, composed of simply modified 

 anal rays ; males at least as large as the females. 



Poecilia Bloch and Schneider. 

 dd. Intromittent organ with many retrorse spines in front and behind ; 

 males highly ornamented and much smaller than the females. 



Acanthophacelus Eigenmann. 

 cc. Caudal peduncle with a knife-like ridge below ; intromittent organ very 

 long, placed under the pectoral, the ventrals of the male under the gill 

 opening ; teeth conical, in a few series. Slender, hyaline. 



Tomeurus Eigenmann. 



Genus Rivulus Poey. 



The new species of Rivulus of British Guiana are best definable by 

 the following key : 



a. Sides without longitudinal markings except a dark band from tip of lower jaw 

 continued as a darker shade of the general color of the side ; head 4 ; depth 

 4|^ ; D. 9 or 10 ; A. 12 ; scales 35 counting to the last one on the caudal ; 

 20-23 scales in front of the dorsal ; eye 3.5, greater than snout ; 1.6 in inter- 

 orbital ; occipital scales as large as the two bordering it on its sides ; caudal 

 broad, subtruncated or rounded, with angular corners above and below ; sides 

 dusky, centers of the scales of the back darker ; middle of caudal with about 5 

 cross-bars ; ventrals margined with dark ; dorsal and anal dark ; origin of anal 

 equidistant from base of middle caudal rays and upper angle of gill opening. 



breviceps. 

 aa. Sides with longitudinal markings. 



b. Anal 14-17. 



c. Head 4-4.5 ; depth 5 ; D. 9 or 10 ; A. 15-17 ; scales 43 plus 4 on the 

 base of the caudal rays ; eye 3.5 in head, equal to snout, 1.6 in inter- 

 orbital ; occipital scale very narrow ; caudal broadly rounded ; origin 

 of anal equidistant from base of middle caudal rays and upper angle 

 of gill openings ; about 30 scales in front of the dorsal ; female with a 



