246 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 21 



don peruvianus Rathbun (1924a). It is now apparent that the differences 

 which heretofore caused them to be regarded as two species belonging to 

 different genera are purely sexual in character, Epialtus peruvtanus being 

 the male, Eupleurodon peruvianus the female, of but a single species, and 

 that an Eupleurodon. 



The evidence consists primarily in a series of 18 individuals from 

 Punta Santa Elena, Ecuador (U.S.N.M. No. 61503), determined by 

 Rathbun as Eupleurodon peruvianus, and containing 5 males, 6 females, 

 and 7 young, plus two ovigerous females from Salinas, Ecuador 

 (U.S.N.M. Nos. 61502 and 70939), specimens obtained by Waldo L. 

 Schmitt. In all the males the hepatic width exceeds the branchial, while 

 in two of the five the preorbital lobe is obsolescent and in the remainder 

 nowhere approaches a tooth. This brings the males into line with Epialtus 

 peruvianus, which, according to Rathbun ( 1925, fig. 53i), has its greatest 

 width at the hepatic level and an orbital arch which forms an angle, not 

 a tooth, with the margin of the rostrum. The seven young specimens, not 

 sexed, show a noticeable excess of hepatic width over branchial, the lateral 

 lobes being reduced to mere tubercles in the smallest. This tendency per- 

 sists in all but the largest of the six females, in which branchial and 

 hepatic width are approximately equal. None of these females is ovig- 

 erous, as was the holotype of Eupleurodon peruvianus Rathbun (1924a). 

 In the two ovigerous females examined (U.S.N.M. No. 70939 and 

 Velero III station 391-35) the branchial width exceeds the hepatic, as 

 required by the description of the female given above. In all females 

 examined, nonovigerous and ovigerous, the preorbital lobe is present and 

 prominent. It is apparent that excess of branchial width over hepatic is a 

 female character attained only at sexual maturity and that the presence 

 of a preorbital lobe is also a female character, its absence in the adult 

 male being associated with the gradual attenuation of the rostrum with 

 age in that sex. 



A 9.5 mm male, the largest specimen examined (U.S.N.M. No. 

 70938), was taken also at Punta Santa Elena, but on the day preceding 

 the series mentioned above. It carries Rathbun's identification as Eupleu- 

 rodon trifurcatus Stimpson, with a question mark, presumably because 

 the hepatic width equals the branchial and there is no preorbital lobe. 

 (Cf. Rathbun, 1925, p. 160, key and diagnosis.) However, when placed 

 with the five males of Eupleurodon peruvianus (U.S.N.M. No. 61503) 

 it completes the series, the slight angle of the orbital arch of the smaller 





