Note U — Chapter IV

There exists, according to Agassiz, only a single species of Dipterus—D. macrelepidotus; whereas four species of Diplopterus have been enumerated—D. affinis, D. borealis, D. macrocephalus, and D. Agassizii. The existence of the last named, however, as a distinct species, is regarded as problematical by the distinguished naturalist whose name has been affixed to it.