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Show 1 9 0 4 .] ASELLOTA-GROUP OF CRUSTACEANS. 3 1 3 in the male of other Isopoda. In Idothea, Sphceroma, Cirolana, Cymothoci, &c., this endopod is generally described as an undivided plate with an " appendix masculina " articulated at its inner margin : this plate, is the first, the " appendix " the second joint of the endopod. This endopod is therefore two-jointed in all Isopoda, Epicaridea and Gnathiidfe excepted; but in most forms only the second joint is transformed, the first being large and lamellar like that of the first or the third pair, while in Asellota and Oniscidse both joints are narrow. YI. The Classification of the Asellota. The tribe or suborder Asellota is very sharply defined from all other Isopoda, but its subdivision into families is a matter of considerable difficulty. As already stated, G. O. Sars in 1897 divided the Asellota into five families, but four of these are far from distinct from each other; moreover, other objections can be raised. His family Desmosomatidae is in reality a rather mixed company: such genera as Macrostylis and Ischnosoma differ strongly from each other in most features; Nannoniscus and especially Ischnosoma are far from being closely related to Desmosoma, &c. I have, for the rest, already, on p. 308, pointed out several difficulties as to these four families; it may be added that from the ‘ Ingolf ' we have several new forms which differ rather or very considerably from the genera of Sars, so that an attempt at arranging them within his families will aggravate the state of things. When nature has not worked out groups well-defined from each other we can of course subdivide a tribe or suborder into families, founding them on some points in the general aspect of the animals, but their number and limitation must then be a matter of personal opinion, and many other authors will propose the establishment of other or of new families not better than those first erected. It is, in my opinion, to be preferred to keep a very large group of genera in the same family, a large number of species in the same genus, than to subdivide respectively the family or genus into families and genera with new names, when sharp lines of distinction are not to be found in nature. It is well-known that differences in the structure of the abdominal appendages are among the most important characters for dividing the order Isopoda into tribes or main-families. That considerable importance must be ascribed to the above-named differences in the structure of the pleopoda in Asellus, Stenetrium, and other Asellota, will probably be admitted, these differences being much sharper than those met with in any other external organ. In the Plankton paper I wrote in 1895 the long passage quoted above on differences in the pleopoda between Asellus and Stenetrium on one side, and several* other genera of Asellota on the other, and continued:-" Es ist anzunelimen, dass alle existirenden Gattungen in die eine oder dieandere dieser zwei nach ausserst scharfen Kennzeichen getrennten Gruppen eingefiigt werden konnen, welche also die 2 Familien bilden, in welche die |