PART 1 



GARTH : PACIFIC OXYRHYNCHA 



371 



9. Anomalous male first pleopods of Galapagos Mithrax species. A. Mithrax 

 (Mithrax) belli, 37.6 mm male {Velero III Sta. 163-34), x 4.05. B. Same, 

 terminal portion, x 12.5. C. Mithrax (Mit/iraculus) nodosus, 38.2 mm male 

 {Velero III Sta. 42-33), x 13.75. 



Remarks: The unusual male first pleopod (see text-figs. 9A, B) 

 deserves special comment. The heavy basal portion appears to be cor- 

 related with the very wide abdomen noted by Rathbun above. The 

 tubular extension which serves as an intromittent organ, unlike anything 

 so far encountered in the family, would at first appear to indicate separate 

 descent. However, the characters which relate the species to the genus 

 Mithrax, the subgenus Mithrax, and the superspecies represented by 

 M. (M.) verrucosus in the Atlantic and by M. (M.) belli in the 

 Pacific are so many and detailed that there is no thought of erecting a 

 separate group on the basis of this aberrant gonopod alone. Rather, it 

 would appear to represent a mutation which has spread to an entire 

 insular population and which would serve as an effective species isolating 

 mechanism should opportunity for interbreeding with mainland species 

 occur. 



