In 1754 and 1756 Gronovius in his Museum ichthyologicum defined the following genera : -

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The Pygidiidce are a family of South American catfishes distinguished externally by the absence of an adipose fin and by the posterior position of the dorsal. Most of them are even more readily distinguished by the presence of spines or thorns on the opercle and interopercle, by twin barbels at the angle of the mouth, and by the absence of all mental barbels. Other characters of the catfishes may be present or absent, and by addition, subtraction, or modification of characters, various subfamilies have been formed. A description of the characteristic structures is given on pages 276-279. The basal habit of all the members of the family is that of burrowing. The opercular and interopercular spines are an adaptation to their habit of insinuation, which is at the root of the commensalism, parasitism, and worse, to which some highly specialized members of the family are addicted. Nematogenys from central Chile, the only representative of the Nematogenyince, is probably more nearly like the ancestors of the Pygidiidce than the other living representatives of the family. It recalls the Siluridce by having a pungent pectoral spine, serrated on its posterior margin, by having but one barbel at the angle of the mouth (the remaining subfamilies having two), by having a pair of mental barbels,

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