Turning our attention now to the species within the genus, instead of the genus as a whole, _Conophis vittatus_ is readily set apart from other members of the genus on the basis of the universal presence of seven supralabials. In basic coloration it also differs, having no stripe on the 1st scale-row, or spots on the venter, and a maximum of four broad stripes on the body. The other species appear to be more closely related; these make up the _C. lineatus_-group. _Conophis nevermanni_ differs so much from the other species that it might be placed in a separate group. Nevertheless, the basic striped pattern, which is masked by the increased melanism of many specimens, indicates that _nevermanni_ is more closely related to the _lineatus_-group than to _vittatus_. The _lineatus_-group, thus, consists of _pulcher_, _nevermanni_ and the three subspecies of _lineatus_. In this group the color pattern is characterized by the high frequency of ventral spotting, darkening of part of the supralabials, dark pigmentation on the 1st scale-row, and more than four dark stripes on the body of adults. _Conophis lineatus concolor_, on which the dark pigmentation on the body apparently is secondarily lost, is an exception. If differences in color pattern be used as an indication of the relationships between the species and subspecies of the genus _Conophis_, I would consider _C. vittatus_ the most divergent unit. The subspecies of _lineatus_ closely resemble one another and, as a unit, resemble _pulcher_ from which they differ primarily in the position of the dorsalmost stripes. _Conophis nevermanni_ is more divergent than is _pulcher_ from the species _lineatus_, but probably is not so far removed from _lineatus_ as is _vittatus_. In the light of what has been pointed out immediately above with respect to resemblances of, and differences between, the species, an hypothesis to account for their formation and for their presence in the areas where they are today is the following: Concurrent with climatic fluctuations in the Late Pliocene and Pleistocene, the northernmost population differentiated into the species _vittatus_, and has subsequently spread north and west from the region of Tehuantepec, México. During the same period _nevermanni_ became isolated in northern Costa Rica.