REPORT ON THE CRUSTACEA MACRURA. 207 



form of chelae, of which the third pair is the longest as it is also the largest. There is no 

 other external anatomical detail or feature of importance that supports this view, 

 whereas there are some very decided ones that bring them both into the same group 

 with the normal division of the Trichobranchiate Macrura, as for instance the tricho- 

 branchiate character of the gills. 



Professor Huxley, in his Memoir, on the Classification of the Crayfishes, 1 was the first 

 to point out that " by the structure of its branchiae Stenopus is sharply separated from 

 Penarns, with which it has hitherto been associated, although it approaches Penseus in 

 the almost complete abortion of the branchial element of the podobranchiao." 



This may be the case in comparison with some species, but the branchial arrangement 

 in several genera of the Pemeidae varies from what has hitherto been considered 

 characteristic of the family. 



The anterior three pairs of pereiopoda characteristic of Penseus is common to all the 

 Astacidae, the only variation being that in Astacus the first pah- is the largest, whereas 

 in Penarns, Stenopus and Spongicola it is the smallest ; in the Astacidea all the legs, 

 especially in the females, have a tendency to form chelae, whereas in the Penaeidea 

 this character never appears posterior to the third pair, and in the Phyllobranchiata 

 never beyond the second. In all the Astacidea the brephalos is in the Megalopa stage, 

 as probably is the case in Stenopus, 2 while that of Spongicola is in the Zoea form. 



If we take the typical forms of the Astacidae, and compare their several parts 

 analytically with those of the genera in the family Stenopidae, we shall find that the 

 variations are of little more than generic importance : — 



The animals generally are subcylindrical. In Astacus the rostrum is flattened 

 horizontally, in Spongicola and Stenopus it is vertically compressed as it is in 

 Phoberus. 



The ophthalmopoda are short in both. 



The first pair of antennae supports two flagella in both. 



The second pair supports a scaphocerite which is short in Astacus, not long in 

 Spongicola, but long in Stenopus. 



The mandible has a two-jointed synaphipod in Astacus, and a three-jointed one in 

 Stenopus and Spongicola. The first pair of siagnopoda in Stenopus has the outer branch 

 single-jointed and reduced, a condition seen in Astacus fiuviatilis and Astacoides 

 madagascarensis. 



The second pair of siagnopoda is almost identical with the same appendage in Astacus 

 fiuviatilis in having the inner lower plates broader than the upper, the reverse of what 

 exists in Astacopsis. 



1 Proc. Zool, Soc. Lond., 1878, p. 780. 



2 Observations of an incomplete character induce me to believe that the brephalos of Stenopus is in the Megalopa 

 stage. 



