Cetopsidium soniae differs from its congeners in the posterior extent of the pelvic fin (falling distinctly short of the vent versus extending beyond the vent and reaching to the origin of the anal fin in C. minutum (Eigenmann, 1912), and reaching to or slightly beyond the anterior limit of the vent in C. ferreirai Vari, Ferraris & de Pinna, 2005, C. morenoi (Fernández-Yépez, 1972), C. orientale (Vari, Ferraris & Keith, 2003), and C. pemon Vari, Ferraris & de Pinna, 2005, the position of the vent (proximate to the base of the anterior most anal-fin ray versus distinctly anterior to that location in C. roae Vari, Ferraris & de Pinna, 2005), the form of the first rays of the dorsal and pectoral fins (with distal filaments present in males versus without such sexually-dimorphic filaments in C. ferreirai), the orientation of the dorsal and ventral profiles of the postdorsal portion of the body (converging posteriorly versus running in parallel in C. orientale), the distribution of dark pigmentation on the body (with pigmentation distributed across dorsal and lateral surfaces of body versus limited to middorsal region in C. ferreirai), the degree of dark pigmentation on the ventral surface of the lower jaw (with

limited or no pigmentation versus with broad band of pigmentation in C. orientale), the form of the pigmentation on the head and body (chromatophores large and stellate versus small and round in C. minutum and C. pemon), the extent of the pigmentation on the dorsal fin (without any pigmentation or at most several small dark basal spots versus with distinct semicircular dark spot basally in C. morenoi, C. orientale, and

C. pemon), the body depth (0.17-0.18 versus 0.14-0.15 of SL in C. roae), and the form of the head in lateral view (triangular overall but relatively deep posteriorly versus more acutely triangular in C. minutum [see Vari et al., 2005).