Physopyxis Cope, 1871

Physopyxis Cope, 1871: 112; described in detail in Cope, 1872: 273;

type-species, Physopyxis lyra Cope, 1871, by monotypy.

Diagnosis. Physopyxis can be distinguished from the other Doradidae by the miniature size (adults less than 31 mm SL) and a unique combination of additional features: extreme development of the exposed pectoral girdle; well developed coracoid process longer than the postcleithral process; anterior plate of the nuchal shield absent; dorsal spine serrated only along the basal portion of its anterior margin; two infraorbital bones in young that become fused into a single plate in adult specimens; pectoral spine reaching to base of anal fin.

Key for species of Physopyxis .

1. Lateral line incomplete, with 5 to 12 lateral plates, each with one retrorse spine; a series of small spines along dorsal midline formed by the external expansion of the vertebral neural spines; adipose fin present (Fig. 2a) .... P. cristata (rio Negro basin)

1'. Lateral line complete; lateral plates well developed to caudal fin and meeting dorsally in larger specimens; adipose fin present or absent (Figs. 2 b-c) ...................................... 2

2. Lateral plates with only one series of spines; superior wing of lateral plates deeper than inferior wing (Fig. 2b); coracoid processes with posterior tip enlarged, knob-like and diverging laterally (Fig. 2e); adipose fin usually present ....................................................................................... P. lyra (Amazon basin in Peru and Brazil)

2'. Lateral plates with two or more series of spines; depth of superior wing of lateral plates equal to or less than that of inferior wing (Fig. 2c); coracoid processes narrow and converging toward central axis of the body (Fig. 2d); adipose fin usually absent ........................................... P. ananas (widely distributed in Amazon and Essequibo basins)