Journal
<p class="rtejustify">A new species of the heptapterid genus <em>Mastiglanis</em>, <em>M. durantoni</em>, is described from three adjacent localities in the upper reaches of the Maroni River in French Guyana. The new species is distinguished from the only other known species in the genus, <em>M. asopos</em>, by several qualitative and quantitative characteristics, such as the slender caudal peduncle (caudal peduncle depth 4.3-5.3% of SL vs. 5.6-6.3%); the upper jaw protruding markedly beyond lower, resulting in long snout (36.5-42.0% of HL vs. 23.4-33.6%); the head depressed (head depth 34.0-44.4% of HL vs. 44.4-53.0%); the procurrent caudal fin rays 12 dorsally and ventrally (vs. 14-17); the dorsal margin of adipose fin concave (vs. variably convex, straight or gently sinusoidal); the occipital process deeply notched, distally V-shaped (vs. distal margin of occipital process variably-shaped, with notch, when present, not deep and forming an obtuse angle); the mesethmoid cornu widest at its midlength (vs. gradually narrower from base to tip). Characters of taxonomic and phylogenetic relevance are observed and illustrated with CT-scan imaging technique. The narrowly endemic and outlying geographical distribution of the new species contrasts with the extremely wide range of <em>M. asopos</em> and suggests a history of peripheral isolation and differentiation. In conformity with that interpretation, the new species has some autapomorphic specializations, while none is currently known for <em>M. asopos</em>.</p>
