18 atheriniDvE: silversides 



v'^. Jaws unequal, the lower more or less projecting (except in 

 C. PROMELAS, in which the premaxillaries are secondarily pro- 

 duced forward) ; rami of mandibles more or less elevated pos- 

 teriorly ; scales large or moderately fine in 27 to about 75 series, 

 their margins entire or crenate ; first dorsal fin beginning over 

 or before orgin of anal ; a conical diverticulum of the air 

 bladder extended backward over one-third to two-thirds of the 

 anal base. Lakes and streams of the Rio Lerma System and 



lakes of Mexico 32. Chirostoma. 



M*. Jaws produced into a short rounded beak ; premaxillaries dilated, 

 but proportionately less widely than in the preceding genera of 

 the subfamily ; origin of first dorsal behind that of anal ; scales 

 small, in about 75 series, their margins entire ; air bladder extended 

 backward a short distance into tail ; rami of mandibles scarcely 

 elevated posteriorly, nearly linear in form. Fresh waters of eastern 



United States 34. Labidesthes. 



t'. Scales lacking on head and most of trunk; jaws pointed anteriorly, 

 the premaxillaries rather narrowly dilated posteriorly ; gape wide, 

 not restricted by membrane between jaws; rami of mandibles sub- 

 linear; belly short; anus in normal position; air bladder broadly 

 extended backward into tail over front of anal base ; first dorsal base 

 behind origin of anal ; scales of moderate size, their margins more or 

 less evidently crenate. Streams of east central Mexico. 



35. Xenatherina. ' 

 s'. Premaxillaries not technically protractile, the skin above continuous 

 with that of the forehead ; a cross furrow at base of premaxillary 

 processes permitting a degree of motion ; rami of mandibles elevated 

 posteriorly ; scales moderately small or fine ; size rather large, com- 

 paratively. Shores and streams of the Pacific slope of America, 

 hh^. Scales on top of head not reversed in imbrication, those of the body 

 larger, in 47 to about 80 series ; vomer without teeth ; anal fin long, 

 with 17 to 24, usually 20 to 23, soft rays ; spinous dorsal well devel- 

 oped. Western shores of North America. 

 ii\ Teeth of jaws simple, conic, in bands ; first dorsal of seven to nine 

 spines. Shores of California, and the outer coast of Lower 



California 36. Atherinopsis. 



ii-. Teeth of jaws forked, uniserial ; first dorsal of four to seven 



spines. Oregon to the Gulf of California. 



jf. Head slightly depressed; gill-rakers 16 to 18 on lower limb of 



outer arch, strongly compressed, curved, serrate on inner 



margins, about one-fourth as long as eye ; inner fork of teeth 



usually much the shorter. Gulf of California. 



Z7. COLPICHTHYS. 



ji^. Head compressed ; gill-rakers 20 to 25 on lower limb of outer 

 arch, slender, terete, straight, nearly smooth, about two-fifths 

 as long as eye ; inner fork of teeth usually little the shorter. 

 Outer coast of Lower California, California and Oregon. 



38. Atherinops. 



hh^. Scales on top of head reversed in imbrication, those of the body 



smaller, in about 75 to 100 series ; vomer with teeth ; anal fin 



shorter, with 13 to 16 soft rays. Western shores and coastwise 



streams of South America. 



