Notes on the status of Clarias ngamensis Castelnau, 1861, C. mellandi Boulenger, 1905, C. prentissgrayi (Fowler, 1930) and C. lamottei Daget & Planquette, 1967 (Pisces, Clariidae) with the rehabilitation of Dinotopteroides Fowler, 1930 as a subgenus of Cl