778 PROF. T. H. HUXLEY ON THE CLASSIFICATION [June 4, 



In Nephrops the branchiae 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 re- 

 duced 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 ru- 

 diment. 



The Branchi&l formula of Axius stirhynchus. 



Somites p do- Arthrobranchiae. Pleuro- 



and their ]a$ad ^ -— VT _ 7 branchiae 



appendages. Anterior. Posterior. 



VII (ep) 0=0 (ep) 



VIII 1 r = 1+r 



IX 1 1 1 0=3 



X 1 1 I 0=3 



XI 1 1 1 1=4 



XII 1 1 1 1=4 



XITI (ep) 1 1 1 = 3 + ep 



XIV 0=0 



5+2ep +5+r +5 + 3 = l8 + 2ep+r. 



The branchial filaments in this genus, again, are arranged only in 

 two series on the stem, though, as they remain filiform, the ap- 

 proximation to the phyllobranchiate type is but slight. 



In Homurus, Nephrops, and Axius the epipoditic divisions of the 

 podobranchiae are ^ery large; but in Thalassina, Stenopus, and 

 Penceus 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 pleurobrauchiae. The podobranchiae of the twelfth and 

 thirteenth somites are reduced to stout curved setose epipodites. 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 arthrobranchiae attached in pairs, from the second 

 maxillipede to the penultimate thoracic limb inclusively. All these 

 branchiae 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 



