The subgenus M. (Scaphigenia) Lacordaire includes six species distributed in arid regions of South America. A systematic revision of this subgenus is presented, including re-descriptions and an update of distribution data of the six species. A preliminary cladistic analysis is performed to test the relationships among the species of the subgenus and if the morphological characters used are suitable. A key is presented to separate the seven subgenera of Megalostomis Chevrolat as well as a key for the species of the subgenus M. (Scaphigenia). Male aedeagus internal sac of the nine studied taxa are described and illustrated. This constitutes the first internal sac descriptions for members of this subtribe and could help clarify the homology of such structures within Cryptocephalinae. M. (S) consimilis Achard is reassigned to the species status on the base of genitalic characters. The cladistic analysis was done using forty-one characters from adult external morphology and male and female genitalia. Two species of two different subgenera of Megalostomis: M. (Megalostomis), and M. (Heterostomis) Lacordaire, and one representative of the genus Themesia Lacordaire were selected as out groups. Results were obtained by implicit enumeration using parsimony software. Three equally parsimonious trees were obtained of 45 steps, Ri=0.952, and Ci= 0.941. Support of each group was evaluated by Jacknifing, Bootstrap and Bremer values. Relationships between species are discussed.