NO. 3582 GENUS ENTOMACRODUS — SPRINGER 31 



also has a relict type distribution. It may be that the distribution 

 of E. decussatus is contracting and will become peripherally limited. 



To explain how a relict species might become limited to the periph- 

 ery, one can draw on the example of the fairy-ring mushroom, 

 which starts from a central area and spreads as an ever enlarging 

 ring around the central area that no longer harbors the species. If 

 the suitable habitat is limited, the mushroom (or fish) will have its 

 last stand along the periphery of the habitat. Matthews (1915) 

 postulated the displacement of primitive forms from a central area 

 by the development of more progressive (successful) forms in the 

 central area. 



A perusal of several recent revisions of Indo-Pacific fishes indicates 

 to me that many species will be found whose distributions in the 

 central and western Pacific are restricted to the periphery of the 

 area. The reason for basing a Pacific peripheral distribution hypoth- 

 esis primarily on Entomacrodus species is that there have been available 

 to me more collections from more diverse localities than have been 

 available to authors of similar ichthyological revisions. 



A peripheral hypothesis for shore fish distribution in the central 

 and western Pacific makes less perplexing such distributions as that 

 of Acanthurus leucopareius. The hypothesis indicates that a species 

 such as Zebrasoma flavescens (Bennett) with a known distribution 

 (completely peripheral) of Marcus, Guam, Saipan, Bikini, Wake, 

 Johnston, the Hawaiian Islands, and Tuamotus (Randall, 1955), 

 need not be expected to occur throughout the central and western 

 tropical Pacific. 



Key to Species and Subspecies of Entomacrodus 



la. Upper lip with crenulae on all or part of ventral margin; gill-rakers usually 

 more than 14 (13 or less only in 8 percent of specimens of E. caudo; 

 fasciatus, 61 percent of E. cymatobiotus, and 50 percent of E. strasburgi). 

 several species commonly exceeding 100 mm SL 4 



lb. Upper lip without crenulae on ventral margin; gill-rakers 9-15 (1 of 228 

 specimens with 18 gill-rakers; 13 or fewer gill-rakers in 93.0 percent 

 of specimens); largest specimen known 49.7mm 2 



2a. Some circumorbital pores included in dark spot on head posterior to eye; 

 segmented dorsal rays 13-15 (uncommonly 15); segmented anal rays 15- 

 17 (uncommonly 17) 3 



2b. No circumorbital pores included in dark spot on head posterior to eye; seg- 

 mented dorsal rays 15 or 16; segmented anal rays 17 or 18. 



macrospilus (Marquesas Islands) 



3a. Supraorbital cirrus of males 5.9-9.5 percent of SL, females 3.9-6.5 percent 

 SL (fig. 11) thalassinus longicirrus (South China Sea) 



3b. Supraorbital cirrus of males 2.3-4.9 percent of SL, females 2.7-4.7 percent 

 SL (more than 4.1 percent in only one specimen, fig. 11). 



thalassinus thalassinus (Indian and Pacific Oceans) 



228-965 O - 67 - 3 



