208 THE VOYAGE OF H.M.S. CHALLENGER. 



The third pair of siagnopoda differs in having the mastigobranchial plate transversely 

 divided by a rib, thus making an anterior and a posterior division in Spongicola and 

 Stenopus, whereas the posterior division alone exists in Astacus ; and the central branch 

 is uni-articulate, while in Spongicola it is two-jointed, and in Stenopus four-jointed. 



In both families the first pair of gnathopoda is six-jointed, and they resemble each 

 other in form. In Astacus the podobranchial plume is developed as part of the mastigo- 

 branchial plate, a feature that is peculiar and, so far as we know, confined to the family 

 Astacidse, whereas in the Stenopidse, the two genera which comprise the family have the 

 podobranchial plumes absent from all the appendages of the pereion except the first pair 

 of gnathopoda, where they are reduced to an almost rudimentary condition and attached 

 at the base only to the mastigobranchial plate. 



The second pair of gnathopoda is pediform in each family, but short and robust in 

 Astacus, and comparatively long and slender in Stenopus ; it carries in both families a 

 basecphysis, which in Spongicola is small and rudimentary, as in Astacoides madagas- 

 carensis, while in Stenopus it is extremely short and feeble. The mastigobranchia is 

 reduced to a rudimentary condition and the podobranchia is wanting in the Stenopidse, 

 while in the Astacidse the podobranchia is developed on and forms part of the 

 mastigobranchial ramus. 



The pereiopoda in the Stenopidse are long, and, with the exception of the third pair, 

 slender, and possess the characteristic Penseid feature of having the carpos longer than 

 the propodos, with the exception of the third pair, which is shorter in Stenopus and still 

 more so in Spongicola. The carpos in all the Astacidse is shorter than the j)ropodos, 

 and the legs have consequently a comparatively shorter and more robust appearance. 



The first pair of pleopoda in the Astacidse is modified in form for sexual purposes in 

 the males of the northern hemisphere and wanting in both sexes in the genera of the 

 southern hemisphere. In the Stenopidse it is uni-branched and foliaceous. 



The second and succeeding pairs in the Astacidse and Stenopidse are biramose, 

 foliaceous, and possess not even the rudiment of a stylamblys ; the posterior pair in 

 the Stenopidse has the outer plates of the rhipidura without a diseresis, and the telson 

 has no transverse division, while in all genera of the Astacidse there is a division or 

 diseresis more or less perfectly defined. 



By thus reviewing and comparing the two families we find that the Stenopidse corres- 

 pond with the Astacidse in the structure of the branchiae, in having eleven pairs of 

 arthrobranchiae, in having five pairs of pleurobranchias in comparison with four in the 

 Australian genera ; also in the form of the mandibles and first two pairs of siagnopoda, as 

 compared with the genus Astacus, and in the subpediform condition of the two pairs of 

 gnathopoda. On the other hand, distinctions exist in the character of the rostrum, 

 which is compressed horizontally in the Astacidse, and vertically in the Stenopidse ; 

 in the scaphocerite being long or moderate in the Stenopidse, and short in Astacidse ; in 



