CARIDEA. 529 



those of other organs posterior to them in position. The constancy 

 of character observed in these parts throughout the Brachyura, amid 

 all the diversities among the species, is evidence of their prime value 

 in classification. Were they less fundamental in their relations, we 

 should find them undergoing modifications like the maxillse, maxil- 

 lipeds, and legs ; for those parts that are lowest in relative rank are 

 those which are most liable to changes. Such are the legs, and the 

 hinder legs for like reason are subject to the widest variations in size, 

 with less differences in other respects than the anterior legs. 



We should, therefore, give a prominence to the peculiarities of the 

 mandibles, in our endeavours to trace out the limits of groups. De 

 Haan, in his work on the Crustacea of Japan, has recognised their 

 importance, and his groups are partly based upon the characters they 

 afford. In this respect we adopt his method, yet with some modifica- 

 tion of his subdivisions. 



The mandibles may be viewed as of four distinct types. 



1. Form slender, simple, much inflexed, with the crown not en- 

 larged or dilated; as in Crangon and Nika (Plate 33), &c. 



2. Form stout, simple, not inflexed, with a broad dentate terminal 

 crown, as in the Penceidce. The organ is placed very obliquely. 



3. Form stout, nearly simple, the crown broad, somewhat divided 

 into a terminal and lateral process, the terminal short and dilated ; as 

 in Atya (Plate 34, fig. 1) and the allied. 



4. Form stout, deeply furcate above, so as to have a terminal and 

 lateral process, each long and narrow ; as in Palcemon, Alpheus, &c. 

 (see Plates 34, 35, 36). 



These forms are characteristic of prominent sections of the Caridea; 

 sections that are well sustained by other peculiarities of structure. 



Besides these peculiarities, the mandibles differ in bearing or not 

 bearing a palpus. This distinction appears at first to be of no less 

 consequence than those pointed out. But it is to be observed, that the 

 portion of a mandible which is of most essential importance in the 

 performance of its functions, is the crown. Differences in this part 

 show difference of habit, and corresponding differences in some points 

 of structure. But the palpus is a small, jointed, accessory appendage, 

 having the same uses as the inner maxillce and not affecting by 

 its presence or absence the capability of the mandible to fulfil its end. 

 This view is sustained by a reference to the species themselves. 

 Among the Palaemons, the organ varies greatly in size, being rela- 



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