206 



U.S. NATIONAL MUSEUM BULLETIN 25S 



Figure 57. — Pleusirus secorrus, new genus, new species, female, 2.5 mm., Campbell station 

 5: a, head and epistome-upper lip complex; b, pereopod 5; c, upper lip. Specimen, station 

 1871: d,e, maxilla 1;/, maxilla 2; g, shape of outer lobe of maxilla 2; h, lower lip. Female, 

 3.5 mm., station 43-B-2: i,j, mandibles; k, maxilliped. 



Pleustes depressa Alderman 

 and 

 Pleustes platypa Barnard and Given 



Figure 58 



Pleustes depressus Alderman, 1936, pp. 56-58, figs. 14-18.— Hewatt, 1946, p. 199 — 



J. L. Barnard, 1954a, p. 9. 

 Pleustes platypa Barnard and Given, 1960, pp. 41-42, fig. 1. 



Pleustes platypa shows some evidence of gene flow with P. depressa 

 in its variable rostrum, ranging from the very elongated shape shown 

 in the type series to the shorter kind shown in the figures herein; 

 nevertheless, the two species remain distinguished everywhere but 

 at the La Jolla locality by the shape of the dorsal segmental teeth 

 which are rounded in P. depressa and sharp in P. platypa. 



The dorsal segmental teeth are highly variable in development in 

 P. platypa as shown in the accompanying table (p. 207). 



To some extent the Pt. Conception zoogeographic boundary holds 

 good for these two species, P. depressa being confined north of the 

 point and P. platypa south of the point, except at La Jolla. Specimens 

 from La Jolla lack dorsal teeth and have rostra similar to P. depressa 

 but all are juveniles 2.6 to 3.4 mm. long. 



A preliminary examination of specimens at hand from Puget Sound, 

 Washington, indicates the extreme plasticity of specific characters 

 and suggests the possibility that the southern species may be linked 

 with those of the Arctic-boreal in a common epigenotype. 



