THE SHORE FISHES OF PERU 237 



This species differs from X. calif orniensis Steindachner in having 

 13 instead of 12 spines in the dorsal fin; and generally in having 12 

 or 13 rays in the anal, instead of usually having only 11. In the 10 

 specimens studied 1 has 11 rays, 7 have 12 rays, and 2 have 13. In 

 8 specimens of X. californiensis 7 have 11 rays and 1 has 12. The 

 scales in X. peruanus are notably smaller, about 70 to 80 (not always 

 easily enumerated because of the small size and condition of the spec- 

 imens) in a longitudinal series, the rows above the lateral line ante- 

 riorly being quite oblique, with about 10 between the lateral line and 

 base of first dorsal spine and about 7 or 8 between it and the base of 

 the first soft ray of dorsal. X. calif orniensis has 60 to 64 scales in a 

 longitudinal series in 8 specimens examined, the rows are parallel with 

 the lateral line, and there are 8 rows between the lateral line and base 

 of first dorsal spine and 6 between it and the base of first soft ray of 

 dorsal. The head is larger, and the maxillary, the spine of ventral 

 fin, and the pectoral fin are shorter in X. peruanus. However, these 

 differences may not be of any importance, as the specimens meas- 

 ured are of very unequal size, those of calif orniensis being much larger, 

 ranging from 125 to 190 mm. in length. The specimens of X. cali- 

 forniensis have about 6 dark longitudinal stripes on the upper half of 

 the side. The small specimens of X. peruanus are plain, with a single 

 narrow dark line on middle of caudal peduncle, extending forward to 

 a point near the vertical from origin of anal where it becomes obscure 

 and disappears. 



The small specimens, serving as type material of X. peruanus, have 

 only two anal spines, the thu'd rsiy although undivided is flexible. In 

 placing the species in this genus it is necessary to assume that the 

 thhd ray will develop into a spine. In all other respects the speci- 

 mens meet the definition of the genus. 



Range. — Lobos de Tierra Island, and North Chincha Island, Peru. 



Family GERRIDAE: Mojarras 



Body elongate to rather short and deep, compressed; head rather 

 small, pointed; mouth usually small, extremely protractile; spine of 

 premaxillaries extending above eyes; maxillary without a supple- 

 mental bone, not slipping under the narrow preorbital; each jaw with 

 small pointed teeth; no teeth on vomer, palatines, or tongue; nos- 

 trils double; gills 4, the membranes separate, free from isthmus; 

 lateral line complete, usually paralleling outline of back; scales large, 

 forming a sheath at base of dorsal and anal; dorsal fin continuous, 

 sometimes deepl}^ notched, with 9 or 10 spines and about an equal 

 number of soft rays; anal with 2 or 3 spines and with about 7 to 9 

 soft rays; ventrals inserted slightly behind pectorals, each with a 

 fairly strong spine and 5 soft rays. 



Two genera are represented in the Peruvian collections studied. 



