94 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



Second air-bladder more or less boot-shaped. 



One hundred specimens, Amazons from Tabatinga to Gurupa. 



Thayeria, gen. nov.i 



Small, elongate Tetragonopterids, reaching a length of nearly 80 mm., dis- 

 tinguished from all others by the unequally lobed caudal. 

 Near Hemigrammus and Creatochanes. 

 Type. — Thayeria obliquus, sp. nov. 



Thayeria obliquus, sp. nov. 



Head about equal to depth, 2.6 (in young)-3 in the length. D. 10^-11; A. 

 16 or 17, rarely 15 or 18. 



Little compressed, elongate, at occiput little deeper than length of head. 



Brassy ; anterior anal lobe dusky ; a light bar from base of upper caudal rays 

 obliquely to tips of the three short rays above the middle ; a dark band from 

 middle of caudal forward ; no humeral spot. 



Many specimens, 47-76 mm. <J, ?., Obidos. 



« 

 Ctenobrycon, gen. nov. 



This genus differs from all other Tetragonopteriuae by its ctenoid scales, which 

 are especially rough on the breast. In other respects it resembles the deeper 

 species of Astyanax. 



Type. — Tetragonopterus hauxwellianus Cope. 



Astyanax asymmetricus, sp. nov. 



Distinguishable from all other members of the genus by the triangular, asym- 

 metrically placed, caudal spot. 



Head 3.3-3.7 ; depth 3-3.3 ; D. 11 ; A. 30, 32, and 28 ; scales 11 or 12-54 or 

 55-59 ; eye 2.5-2-75 in the head ; interorbital 3.1-3.75. 



Compressed, symmetrically elliptical to the slender caudal peduncle. Occipital 

 process about one-fourth the distance of its base from the dorsal. Snout long and 

 pointed. Maxilliary long and slender, about as long as the eye. Three or four 

 teeth in front row of premaxillary, — if four the third is out of line ; five teeth in 

 second series, the denticles arranged in a nearly straight line ; two very minute 

 teeth on the maxillary. Mandible half length of head. 



All but the tips of the middle caudal rays black, the spot continued obliquely 

 downward on end of caudal peduncle to its lower edge, otherwise immaculate. 



Three specimens, 40 to 51 mm. long. Tabatinga. 



1 In memory of Nathaniel Thayer, through whose liberality most of the species 

 described in this paper were collected. 



