This genus is named from _προσωπὶς_, apparently in allusion to its seemingly masked face, most of the species having yellow markings more or less conspicuous upon the face. It is the least pubescent of any of the bees, even less so than those confirmed parasites, the genera _Nomada_ and _Stelis_, thus further tending to corroborate its apparently parasitical habits, for none of the truly pollinigerous bees are so destitute of hair. The ground-colour of the species is intensely black, variously decorated on the face, thorax, and legs, with markings of different intensities of yellow; but one of our species, the _P. variegata_, is also gaily marked with red. Indeed exotic species, and especially those of warm climates, are often very gay insects. They have usually been considered as parasitical insects, from their being unfurnished with the customary apparatus of hair upon the posterior legs, with which pollinigerous insects are generally so amply provided. In contradiction to their parasitism, it is asserted that they have been repeatedly bred from bramble sticks; this circumstance is no proof of the fact of their not being parasitical, for many bees, for instance _Ceratina_, _Heriades_, etc., nidificate in bramble sticks, and they may have superseded the nidificating bee by depositing their ova in the nests of the latter; although it certainly is a remarkable circumstance that some one of these bees has never escaped destruction in the several instances in which these have been thus bred. It is also said that their nests contain a semi-liquid honey. The fact of the larva of a wild bee being nurtured upon any other provender than a mixture of pollen and honey, does not elsewhere occur, and it would seem to contradict the function this family is ordained to exercise, by conveying pollen from flower to flower, and which besides, in every other case, constitutes the nutritive aliment of the larva. But then, again, the structure of its tongue, which resembles somewhat that of _Colletes_ in lateral expansion, and with which it would be provided for some analogous purpose, seems to contradict parasitical habits, although St. Fargeau asserts that it is parasitical upon this genus, and if so, although it has not been observed in this country, the analogous structure of the tongue might be perhaps explained.