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"The full definition drawn up by Professor Sars was probably based on the species L. lignorum alone. To include the other species undoubtedly belonging to the genus it requires some slight modification. Thus the epipod of the maxillipeds is not always lanceolate, and the outer ramus of the uropods is not always unguiform. Authors generally (myself included) have agreed in speaking of the trunk limbs as similar in structure, but, while no doubt they have some features in common, especially in regard to the terminal couple of joints, they are at the same time uncommonly well provided with distinguishing points. Between the first gnathopod and the fifth peraeopod the contrast is sufficiently striking.
In the well-known species of the Atlantic coast Harger notices that in the mandibles ' below there is a slight tubercle, apparently the rudiment of the molar process.' His conjecture is supported by the rather stronger development of this tubercle in L. segnis.
Between the acutely lanceolate epipod of the L. Ugnorum, and the forms with rounded apex in L. segnis and L. pfefferi, the narrow leaf shape in L. antarctica offers an intermediate term. In his elaborate description of the last-named species Dr Pfefifer broaches an extraordinary theory that ' the pleopods in general have not the value of a limb but of an epipod, so that accordingly the branchial plates of the Isopoda Like those of the Decapoda are epipods, and therefore in a certain sense equivalent formations.' That this view has met with no acceptance was to be expected." (Stebbing, 1904)