Parenti: Myers (1958) stated that the genus Aplocheilus represents the most basic characteristics of cyprinodontiform fishes which have either been lost or become more derived in other genera. These characters include the arm of the premaxilla being free rather than embedded in the skin on the side of the head, and the swimbladder extending through several hemal arches rather than ending at the first hemal arch.

The Old World aplocheiloids are distinguished from all other killifishes by the fusion of the posttemporal and supracleithrum (fig. 7C) to form one slender bone connecting the shoulder girdle to the skull. The fusion is complete, and no joint lines are visible. In the Neotropical aplocheiloids, the posttemporal and supracleithrum are similarly shaped, however, the two bones may always be separated easily.

Gill Arches: Two derived characters of the gill arches distinguish the Old World aplocheiloids. One is the reduction of the basihyal to a small triangular-shaped bone which is capped by a large cartilaginous wedge (fig. 11 A). As stated, in all aplocheiloids, the basihyal is very wide, and forms the basis of a wide "tongue" which is visible upon opening the mouth. In the Neotropical aplocheiloids, as well as in the cyprinodontoids and other atherinomorphs, the basihyal is ossified for more than half its length.

SUMMARY OF DERIVED CHARACTERS
1. Supracleithrum fused to posttemporal.
2. Small, triangular basihyal capped by a wedge of cartilage.
3. Interarcual cartilage attached directly to lateral face of the second pharyngobranchial.
4. Premaxillary ascending processes tapered.

In a recent paper by Radda (1977), four new subgenera are named, each to encompass a group of species referable to Aphyosemion. Radda included a phylogenetic tree which purports to summarize the relationships of subgenera within the genus. The monophyly of Aphyosemion is doubtful, following Radda's diagram, for the subgenus Pronothobranchius and the genus Fundulosoma, considered here as close relatives, are included as more closely related to some subgenera of Aphyosemion than they are to each other. Furthermore, the genus Nothobranchius is not considered at all; therefore, it is unclear whether the implication is that Nothobranchius is in turn most closely related to Fundulosoma or Pronothobranchius, to some subgroup of Aphyosemion, or whether it need be considered at all in a revision of Aphyosemion. The genus Aphyosemion is large, currently comprising over 110 species; if some members are more closely related to Nothobranchius species then the two genera must be considered together in a phylogenetic analysis. If the two genera do not form a monophyletic group, then Nothobranchius need not be considered in a study of the interrelationships of Aphyosemion and its subgenera. The problem of defining monophyletic genera and subgenera extends to all members of the Old World aplocheiloids. Scheel (1972) has recommended that the genera Epiplatys and Aplocheilus be synonymized. Clausen (1967) has named a new subgenus of Epiplatys to included the species E. duboisi; a subgenus Aphyoplatys is named to indicate that this species is intermediate between Aphyosemion and Epiplatys. Wildekamp (1977) has recently named a new subgenus of Nothobranchius to include the species N. janpapi; it is named Aphyobranchius to reflect its intermediacy between Aphyosemion and Nothobranchius. It is of little use, however, to know that all the nominal genera of Old World aplocheiloids grade into one another from the Aplocheilus type to the Nothobranchius type. Logically, there is no reason not to classify all the species in one genus; however, that too would be avoiding the problem of the interrelationships of the included species as much as if each species were put into its own genus.