16 ATHERINID^ : SILVERSIDES 



bb'. Anus advanced in position, as in Atherina ; air 

 bladder extended backward to beyond middle of 

 anal base ; gape very strongly arched ; teeth of 

 jaws in two series, those of the outer premaxil- 

 lary series enlarged and directed forward more 

 than downward. Streams of east central Mexico. 



21. Archomenidia. 



00'. Scale margins strongly crenate or roughly laciniate ; 



mandible oblique, the outline of head angulated at its 



base ; mouth very small, the maxillary not nearly 



reaching to below front of eye, and the length of 



mandible before corner of gape being only about as 



long as the pupil; lower jaw included; air bladder 



not extended into urosome. 



cc\ Head not compressed, its width being equal to its 



depth ; body more nearly terete, the belly very 



broadly rounded ; base of spinous dorsal in front 



of vertical from origin of anal ; dorsal and anal 



fins wholly naked, the scales of the body fitting 



tightly against the "base of the rays ; scales thinner, 



with well developed apical radii, scale margins 



crenulate or more evenly laciniate. Tropical Pacific. 



22. HUBBESIA. 



cc'. Head and body strongly compressed ; base of 

 spinous dorsal entirely or in part over front of 

 anal ; dorsal and anal fins with a sheath of large 

 deciduous scales ; scales thicker, with the apical 

 radii obsolete, and the margins coarsely and irregu- 

 larly laciniate. Tropical Atlantic, north to New 



York , 23. Membras. 



2*. First dorsal fin beginning notably behind origin of anal ; 



pectoral fin falcate, usually longer than the head ; belly 



more or less sharply compressed, as though pinched 



between thumb and finger ; body of peculiar form, the 



anterodorsal and posteroventral, and the posterodorsal 



and anteroventral contours, respectively, being nearly 



parallel; air bladder broadly extended backward into 



tail over first fourth or third of anal base ; mandible 



oblique, the outline of the head angulated at its base. 



dd^. Scale margins entire, or in part weakly and coarsely 



crenate, the apical circuli obsolete ; belly shortened, 



the origin of the anal fin being nearer to the head 



than to the base of caudal ; end of second dorsal a 



little in advance of that of anal ; rami of mandibles 



little elevated posteriorly (except in T. sardina). 



Western Mexico and Central America to Colombia 



and Brazil 24.Thyrina. 



dd"^. Scale margins all finely laciniate, produced outward 

 between the ends of the well developed apical radii ; 

 origin of anal fin midway between head and base of 



