326 FISHES OF WESTERN SOUTH AMERICA 
The complete identity of our specimen with the type of Oe. exodon is not certain 
in the absence of scale counts and measurements, and considerable doubt may be 
entertained from the fact that the type has 193 or more anal rays, while our speci- 
men has but 163; the anal is conspicuously elevated in our specimen (to about 
three-fifths the greatest body depth) and even more so in the type (two-thirds). 
The dimensions of the former are much as we might expect in a juvenile of the 
latter. 
Head 7.5; depth at the pectoral fin equal to the length of the head; A. 168, its 
origin under the angle of the preopercle, much in advance of the pectoral fin; anus 
under the eye; width of the head about 3 in its length; interorbital greatly arched; 
eye without free orbital margin, 3.5 in the snout, 9.33 in the head; scales on sides 
above the anal fin-musculature larger, about 70 in a longitudinal series; the back 
in front of the dorsal filament naked, with minute pits; scales covering the anal 
musculature minute. 
Naked portion of the back dusky; sides light, no distinct markings. 
In general appearance this is a Porotergus, and so closely resembles Sternarchogi- 
ton natterer’ (Steindachner) that it would be easy to mistake it for an abnormal 
specimen of that species. The head, however, is decidedly different, the eyes are 
less elevated, the mouth different, and the teeth unique. 
Known only from Iquitos and the nearby Rio Ambyiacu region. 
Genus 183: ADONTOSTERNARCHUS Ellis 
Adontosternarchus Ellis, 1913, Mem. Carnegie Mus., VI, 155. 
Type: Sternarchus sachsi Peters 
Throughout Amazons, the Madeira, and the Orinoco 
Characterized by the heavy lower jaw into which the pointed upper jaw fits 
by a V-shaped groove, somewhat as in the case of the whalebone whale; edentulous. 
KEY TO THE SPECIES OF THE GENUS ADONTOSTERNARCHUS 
a. Without longitudinal stripes; colors variegated, at least while in fresh condition........ sachsi 
aa. With a pale longitudinal striping.......... eee, Me ene egearety ee Pee dr anes balaenops 
456. ADONTOSTERNARCHUS SACHSI (Peters) 
Sternarchus sachsi Peters, 1877, Monatsb. Akad. Wiss. Berlin, 473, fig. 4, Venezuela; 
Eigenmann and Eigenmann, 1891, Proc. U. 5. Nat. Mus., XIV, 62. 
Sternarchogiton sachsi Kigenmann and Ward, 1905, Proc. Wash. Acad. Sci., VII, 165; 
HKigenmann, 1910, Rept. Princeton Univ. Exped. Patagonia, III, 448. 
Adontosternarchus sachsi Ellis, 1913, Mem. Carnegie Mus., VI, 156, lower Amazons; 
Fowler, 1939 (1940), Proce. Acad. Nat. Sci. Phila., XCI, 278, one, 154 mm., Contamana. 
Rivers Orinoco, upper and lower Amazon, Madeira 
15719, 4, the largest about 195 mm., Iquitos, Allen, September, 1920. 
These specimens while fresh show considerable variegation in pattern, no trace 
of a longitudinal striation; distal half of the pectoral fin, and the distal portion of 
