10 CHARLES MIDLO AND HAROLD CUMMINS 



nearly complete in the hand. Perodicticus and Galago, how- 

 ever, are no further advanced than Loris with respect to this 

 character. Granting Hill's generalization, which is based on 

 other morphologic criteria, it appears that the extent of ridge 

 formation may follow a more or less independent evolutionary 

 course, perhaps because it is subject to a control by local 

 functional factors. 



The diversity of prosimians recalls the variability among 

 marsupials described by Dankmeijer, though there is no genus 

 presenting the extreme simplicity expressed in some mar- 

 supials by the complete absence of ridge formation. The range 

 of prosimian variation finds also a parallel among the simians, 

 though in that group complete ridge formation is the rule 

 rather than the exception. 



With reference to the extent of ridged skin, Aotus is the 

 least advanced of all simians which we have examined. The 

 apices and slopes of pads display fully formed ridges, but 

 in other regions of the palm and sole only islands occur, 

 commonly elongated and aligned in such a way as to suggest 

 that but slight structural advance would bring about their 

 conjunction end to end as ridges. In some other platyrrhines 

 (Saimiri, Oedipomidas, Leontocebus, Seniocebus, Cebus) by 

 far the greater area of the palm and sole is ridged but at least 

 some individuals of these genera have regions lacking com- 

 plete ridge formation. Such regions, in the palm, are in the 

 distal part of its central portion (coinciding with the simian 

 line). In the sole it is less commonly the central area which 

 is thus distinguished, but the proximal heel region instead. 

 The remaining platyrrhines and all catarrhines, except rarely 

 in the Colobinae, have continuously ridged palmar and plantar 

 surfaces, though even here epidermal ridges are often less 

 distinctly and regularly constructed in the flexion furrows. 



There are suggestive indications, in those platyrrhines ex- 

 hibiting regionally incomplete ridge formation and in the 

 Colobinae, that the foot is farther advanced than the hand 

 toward the state of a completely ridged surface. Such greater 

 advance is indicated by the fact that in the sole areas lacking 



