These institutions have numerous specimens of small Amazonian eleotrids, many of them less than 10 mm standard length, but apparently all are Microphilypnus.
However, the pelvic fins are widely separated at the base, as usual in eleotrids, and there is no vestigial median connecting membrane between them, as seen in Gobiidae with secondarily separated pelvic fins.
The only eleotrids hitherto known from the interior of the Amazon basin belong to the genus Microphilypnus Myers 1927, and also are quite small.
Diagnosis: Leptophilypnion differs from all other eleotrids in its extremely small size (all known specimens less than 10 mm standard length) and in having only 5 instead of 6 branchiostegal rays.
Its cephalic laterosensory canal system is among the most reduced observed in eleotrids. Leptophilypnus flu viatilis and L.
Cephalic neuromasts usually are readily visible in gobiids and eleotrids including Microphilypnus and Leptophilypnus and their pattern may be helpful in recognition of taxa and their classification.
Compared to the cephalic neuromasts of Microphilypnus, Leptophilypnus, and other eleotrids, those of Leptophilypnion are much fewer and less organized.
Diagnosis: Minute stout-bodied eleotrids with large ctenoid scales confined to posterior three-quarters of body (commencing after end of first dorsal fin but before origin of second dorsal fin), and no cycloid scales; head and body anterior to origin of second dorsal fin scaleless.
Leptophilypnion seemingly is most closely related, not to the Amazonian species of Microphilypnus, but to the Central American estuarine and freshwater eleotrids of the genus Leptophilypnus Meek and Hildebrand 1916 (type species Leptophilpynus fluviatilis Meek and Hildebrand 1916).