Classification of the Decapod Crustaceans. 459 



II. 



1. The earliest of tlie surviving classifications of tlie 

 Decapoda is that established by Latreille in 1806 *, in which 

 the order is subdivided into Mackura or " tailed '■* forms and 

 Braciiyura or Crabs. Rougiily speakinj^, this division 

 depends on the condition of the abdomen, which in the 

 Macrura is carried at length and in the Brachyura is folded 

 under the thorax. In framing a definition, however, it is not 

 possible to rely on the above criterion, for in the Porcellanidae, 

 the Hippidea, and the Lithodidce, which are undoubtedly 

 nearly related to tailed forms, the abdomen is carried as in 

 the Crabs. The absence from the Brachyura of the limbs of 

 the sixth abdominal segment is a better character of seffara- 

 tion, but even this breaks down in tiie case of the Lithodidas, 

 which were, indeed, placed by Latreille with the Crabs. 

 Another criterion which is all but absolute is given by the 

 fusion of the carapace at the side to the epistome. This is 

 found in the Crabs, but only in the Scyllaridea and Eryonidea 

 among the Macrura. No single difference, however, can be 

 found which will absolutely and sharply define the Brachyura 

 from the Macrura. 



2. The next important step in the working out of the 

 system was the establishment by II. Milne-Edwards in 1834 

 of a third suborder, tiie Anomura, intermediate between the 

 two of Latreille. In the new group were placed certain of 

 the higher Macrura (Paguridaj, Hippid^e, Porcellanida3) and 

 lower Brachyura (Dromiidaj, Homolidse including Lithodes^ 

 Raninida^), the abdomen in all these forms being more or less 

 modified from the primitive macrurous condition, but keeping 

 the sixth pair of limbs, except in the last two families. 

 Milne-Edwards's Anomura has had a chequered history in 

 the hands of various authorities, having been alternately 

 added to or reduced, retained or parcelled out again between 

 the Brachyura and Macrura. In a recent paper f I have 

 tried to show that the macrurous members of the original 

 suborder, with the addition of the Galatheinea and Thalas- 

 sinidea, form a natural group, and must be retained as such 

 in the classification. 



3. The last important proposal for the modification of tiie 

 classification of the Decapoda was made by Boas in 1880 \. 

 On the basis of an examination of the anatomy of a number 



* fien. Crust. Insoct. i. Fabricius's two classes Kleistagnatha and 

 Exochnata of " Insects," and Lamarck's Cancri brachyuri and Cancri 

 macrouri, had much the same extension. 



f Gardiner's ' Fauna of the Maldives,' vol. ii. p. 6!J0. 



t Kongl. Danske Vidensk. Sclsk. bkrifter, (6) i. p. 23. 



32* 



