766 PROF. T. H. HUXLEY ON THE CLASSIFICATION [Julie 4, 



the last thoracic somite. There is also an anterior series of six 

 arthrobranchiae, and a posterior series of five (p. 760, fig. 4, arb), 

 disposed as in Astacus. But instead of the two rudimentary pleuro- 

 branchiae borne by the twelfth and thirteenth somites, in Astacus, 

 there are three perfect branchiae attached to the eleventh, twelfth, 

 and thirteenth somites (fig. 4, n~i3plb), in positions which, in the 

 case of the two latter, exactly correspond with those of the rudi- 

 mentary pleurobranchiae of Astacus. 



The pleurobranchiae and the arthrobranchise are very similar to 

 those of Astacxis ; but the inner surface of the stem is channelled by 

 a sort of groove, the edges of which are pretty sharp. I could find 

 no hooks, nor spines, nor hooked papillae on any part of these 

 branchiae. 



The podobranchige (fig. 5, i, u, in) resemble those of Astacus 

 in their general form ; but the basal portion is relatively larger aud 

 the stem stouter, while, at first sight, the lamina appears to be 

 altogether wanting. Close examination, however, shows that the 

 apex of the branchia is distinguishable into an apical plume and a 

 lamellar appendage, but that the latter is relatively very small ; in 

 fact, it is a mere outgrowth of the inner wall of the stem, and is 

 neither bilobed nor plaited. Moreover its surface is beset with 

 numerous filamentous prolongations, which are altogether similar to 

 ordinary branchial filaments, except that the extremity of each, 

 instead of being smooth and rounded, is provided with a short 

 recurved hook-like seta (fig. 5, n a, iv). The groove on the inner 

 or posterior face of the stem is broader than in Astacus. It is 

 bounded on the inner side by a decurrent fringe, which runs down 

 onto the base and stops there, as in the foregoing genera (fig. 5, n). 

 This fringe is provided with long slender curved setae, each beset 

 with imbricated scales and terminated by a recurved hook (fig. 5, v). 

 The external ridge is wider than in Astacus, and is provided with 

 similar hooked setae. It may be said, in fact, that the whole lower 

 half of the posterior margin of the stem in Astacopsis has the character 

 of the posterior margin of the base, while in Astacus there is a 

 sharp demarcation between the base and the stem. 



In principle, therefore, the podobranchiae of Astacopsis are similar 

 to those of Astacus ; and the main difference between the two is that 

 the lamina in the former is represented by a slight expansion of the 

 stem, which is neither bilobed nor plaited, while its surface is covered 

 with cylindrical filaments terminated by hooked spines. In Astacus, 

 as in Cambarus, on the other hand, the lamina is large, bilobed, 

 plaited, and the place of the filaments is taken by mere papillae 

 terminated by similar hooks. Moreover, in Astacus and Cambarus 

 the setae of the base are not hooked. 



The appendage of the first maxillipede is similar to that of 

 Astacus in form ; but on the outer surface of the outer lobe there 

 are sixteen or eighteen short branchial papillae, some of which, but 

 apparently not all, are terminated by hooks ; in fact, except in size, 

 they quite resemble the filaments of the other branchiae. 



In this Crayfish, therefore, the first podobranchia is not reduced 



