MACROURA. 499 







pertain either to the group Astacidea, already pointed out as of supe- 

 rior rank among Macroura, or to the Thalassinidea, a group ranging 

 towards the same high rank in some of its characteristics, as shown in 

 the longitudinal sutures of the carapax. Throughout both these 

 groups, the scale is either wholly absent, or of comparatively small 

 size. 



e. Operculiform outer maxillipeds are seldom to be found among 

 the Macroura. They occur in a genus of Thalassinidea Callianassa; 

 also in the genus Gnathophyllum among the Garidea, and less per- 

 fectly in Pontonia. In either case the approximation is but slight to 

 the true Brachyural form, in which these organs are not only lamellar, 

 but are fitted neatly to the epistome and lateral margins of the buccal 

 area. 



In this review we have indicated the separation of two groups from 

 the other Macroura, namely, the Thalassinidea and Astacidea. They 

 approach, as shown, most nearly to the Brachyura. It remains to 

 consider, whether the remaining species constitute a single group or 

 more than one. The principle upon which we look for a farther sub- 

 division, rests still on a divergence from the Brachyural structure, 

 that is, an inferior state of concentration in the nervous system as 

 indicated in the structure of the species. 



There is no peculiarity of the Brachyura more striking and more 

 uniform than that of the position of the strong chelate legs. They 

 are invariably the anterior pair. This, therefore, is one of the marks 

 of that superior force which belongs to the anterior part of the cephalo- 

 thorax, that is, of that more highly organized nervous system, which 

 gives to the anterior part of the body its superior or subcephalic cha- 

 racter. Among the Macroura, the diffusion of the forces is apparent 

 not only in the largely developed abdomen, the presence of natatory 

 abdominal appendages, the prolonged thorax, and elongated antennae 

 without fossettes, and eyes without orbits; it is also seen in the 

 backward transfer of power among the legs. The anterior pair 

 retain their supremacy in the Astaci and Thalassinidea, although 

 we find a following pair, or two, sometimes small chelate. Among 

 the other Macroura, the same fact is observed, as, for instance, in Al- 

 pheus ; but often, as in Palaemon and others, the second pair is the 

 larger and more powerful pair. These two classes of species those with 

 the first pair strongest, and those with the second pair so are so 



