4 PROCEEDINGS OF THE NATIONAL IVIUSEUM vol. ii9 



series extending to the posterior midbase of the hypural fan. Vertical 

 scale rows are counted between the anterior parts of the dorsal and 

 anal fin bases. 



Although the Nannostomina as a whole are easily definable and 

 the species readily separable, their segregation into generic groups 

 having a clear phylogenetic basis has not proven easy. However, 

 some indication of their possible phylogenetic history is provided by 

 their morphology, and morphological definitions of two generic groups 

 is possible. Of the characters that seem to have generic and specific 

 significance, the following appear to be most important. 



Anal fin : The anal fin of males of this sub tribe is used as an accessory 

 sexual organ to guide the sperm toward the female's vent. During 

 the spawning act, the anterior, posterior, and distal parts of the fin 

 are cupped to form a bowl that partially covers the female's vent, 

 apparently directing the sperm toward the eggs as they are laid. 

 Correlated with this function is a previously overlooked morphological 

 fact. The individual anal fin rays of the males of several species are 

 widened in the sagittal plane (figs. 4 and 6). This modification is also 

 found to a certain extent in some members of the sub tribe Pyrrhu- 

 linina, the closest living relatives of the Nannostomina. This feature 

 seems to have more significance at the specific than at the generic level. 



Infraorbital bones: The usefulness of these structures also has 

 been overlooked by previous authors. There are two types of infra- 

 orbital bone arrangement in the subtribe. In one they are more 

 elongate, and both the first and second elements have a bony infra- 

 orbital canal. In the other type, the first two infraorbitals are 

 short and the second is \\dthout a bony canal. The closest relatives 

 of the Nannostomina, the Pyrrhulinina, have a canal in both the first 

 and second infraorbital bones; its absence in the second infraorbital 

 of some Nannostomina is possibly a neotenic specialization. This 

 character is considered here to have generic significance because it 

 is consistent and correlated mth a definite difference in snout and 

 head shape. 



Color: The melanophore patterns occurring in this subtribe can 

 be separated conveniently into three categories. 



The first of these consists of long, dark, horizontal stripes present 

 in almost all species. The term stripe is restricted in this account to 

 the elongate pigmented areas that extend in a horizontal direction 

 on the sides of these fishes. There are several of these stripes as 

 foUows: Primary stripe, the main midside stripe usually extending 

 from the snout to the eye, across the operculum to the lower part of 

 the caudal peduncle and onto the caudal fin. Secondary stripe, the 

 dark stripe above the primary stripe. It usually extends from the 

 nape or the top of the head to the upper portion of the caudal peduncle. 



