328 FIELD MUSEUM or NATURAL HISTORY ZOOLOGY, VOL. X. 



the basal half of the dorsal and caudal fins black, the distal half of the 

 latter often yellow, a condition not conspicuous on west coast specimens. 



The fresh water individuals are much more uniform in color, and 

 usually smaller than those from salt and brackish water. Those taken 

 from fresh water might be regarded as a variety under the name Mol- 

 lienisia sphenops gillii (Kner & Steindachner). 



This species is represented by numerous specimens in the present 

 collection. The largest females are 100 and the largest males 78 mm. 

 in length. It occurs abundantly in quiet ponds and arms of the rivers 

 and creeks of both slopes of Panama, but not in the Rio Tuyra Basin. 



The males of this species are so large that they are pretty certainly 

 caught along with the females and do not escape through the meshes of 

 the net, as has been stated of other viviparous species of this family, 

 in explanation of the small number of male specimens occurring in col- 

 lections. For this reason we have counted 2,142 specimens collected 

 at various points and on various dates during two periods of the two 

 years during which these collections were made. Among this number 

 there are 579 males or nearly 3.7 females to every male. The disparity 

 of males to females is, however, believed, in reality, to be greater than 

 these figures indicate, for this species was many times taken in such 

 abundance that only a small number of them was preserved. In such 



FIG. 10. DISTAL PART OF INTROMITTENT ORGAN OF M oUUnisia spkenops 



(Cuvier & Valenciennes). (Greatly magnified.) 



cases, due to the fact that the males are more brightly colored and thus 

 more conspicuous, it is quite probable that more than their proportion- 

 ate number was selected, although we do not remember that any par- 

 ticular attention was given to this matter at the time. 



Habitat: Coasts and rivers from Sinaloa, Mexico, both slopes of 

 Panama, south to eastern Colombia, Venezuela and the Leeward 

 Islands. 



