500 CRUSTACEA. 



related in other characters, that no line of demarcation can properly 

 be drawn between them. It appears to be upon this level, admitting 

 of a vibration of force between these two pairs, that the numerous 

 modifications occur among the typical Macroura; for, while the larger 

 second pair, in certain genera, might imply a lower grade, other cha- 

 racters come in as a counterpart to raise the type to a level with 

 those species having the first pair largest, and the gradations from the 

 one to the other are by insensible shades. There is, therefore, no 

 natural subdivision based on this difference of structure. 



But this backward transfer of force, or diffusion among the poste- 

 rior ganglia, takes also another step among the Macroura, so that in 

 certain species, the cephalic part of the body is still less of a head 

 than in those before referred to. The power of the arms, instead of 

 resting between the first and second pairs of thoracic legs, passes to a 

 pair still more posterior, the third pair; and this third pair is not 

 feebly chelate a fact true of the same legs in Astacus, but it is the 

 strongest and longest pair, to which the two anterior are subsidiary 

 and not in any sense superior. This is the case in the Pensei. This 

 characteristic may, therefore, serve to divide the Macroura, exclusive 

 of the Astacidea and Thalassinidea, into two groups, the CAEIDEA and 

 PENJEIDEA. 



There are species still lower (Acetes and the allied), in which all 

 the legs have the feebleness of those in Mysis ; even the third pair is 

 not chelate, or only obsoletely so. The anterior legs have no higher 

 value or functions than the posterior. These are properly the lowest 

 of the Macroura, and constitute a division of the Penoeidea. Some of 

 them have even one or two posterior pairs of thoracic legs wanting, 

 as in some of the Mysis group, and in the Entomostraca. 



On these grounds, we institute four grand divisions of the Macroura. 

 We leave it for others to decide whether or not the Thalassinidea 

 should be divided into a higher and lower group, equivalent to the 

 other groups here adopted, on the ground of the branchiae being solely 

 thoracic or partly abdominal appendages ; if so, the number of groups 

 would be five, three in the typical line, and two in the aberrant. 



The four groups may be characterized as follows : 



I. THALASSINIDEA. Carapax duabus suturis longitudinalibus subdivi- 

 sus, ssepeque sutura dorsali transversa. Antennae externaa squama 

 basali nulla vel parva instructae. Pedes 6 postici directione non 

 consimiles ; antici longiores et crassiores, fossorii et ssepius chelati. 



