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Show 778 PROF. T. H. HUXLEY ON THE CLASSIFICATION [June 4, In Nephrops the branchia? have the same structure, but the branchial plume of the podobranchia of the second maxillipede is absent or rudimentary; hence the number of the branchiae is reduced to 19. In Axius the podobranchia of the penultimate thoracic limb is reduced to an epipodite by the disappearance of the branchial plume, and the hindermost pleurobranchia is also wanting. The arthro-bianchia of the second maxillipede is represented by a mere rudiment. The Branchial formula of Axius stirhynchus. Somites O W fl *f Vl Pl X* il I ll_l LllCll appendages. VII VIII IX X XI XII XIII XIV Podobranchia?. 0 1 1 1 1 1 0 0 (ep) (ep) Arthrobranchia?. Anterior. 0 r 1 1 1 1 1 0 Posterior. 0 0 1 1 1 1 1 0 Pleurobranchia?. 0 0 0 0 1 1 1 0 - = = = = = 0 (ep) 1+r 3 3 4 4 3 + ep 0 5 + 2 ep + 5 + r + 5 + 3 =l8 + 2ep+?\ The branchial filaments in this genus, again, are arranged only in two series on the stem, though, as they remain filiform, the approximation to the phyllobranchiate type is but slight. In Homarus, Nephrops, and Axius the epipoditic divisions of the podobranchia? are Aery large; but in Thalassina, Stenopus, and Peneeus they are much reduced in size, and the branchial element disappears in more than the hindermost. In Thalassina scorpioides the last thoracic somite is provided with no branchiae of any kind, nor have its limbs any epipodite, and there are no pleurobranchia?. The podobranchiae of the twelfth and thirteenth somites are reduced to stout curved setose epipodite3. In the eleventh there is a similar epipodite, but a branchial plume springs from its base. The tenth had a small epipodite, without a branchia in the specimen examined; but I am inclined to think that the branchia may have become detached ; for the similar epipodite of the external maxillipede bore a mutilated, or rudimentary, small branchia. The small epipodites of the second maxillipedes figured by Milne-Edwards were broken off; but the places to which they were attached were discernible. The first maxillipede had no epipodite, in which respect Thalassina approaches Callianassa. There are twelve arthrobranchia? attached in pairs, from the second maxillipede to the penultimate thoracic limb inclusively. All these branchia? are remarkable from the fact that, for a greater or less distance from the base of the gill, the stem is provided with broad imbricated foliaceous expansions, which are traversed by ramified |