DECAPODA. 391 



The order Dccapoda includes the highest forms of the 

 entire class of the Crustaceans, such as the Lobsters, Hermit- 

 crabs, and Crabs, and it is divided into the following three 

 tribes : 



TKIBE I. MACEURA. The members of this tribe, such as 

 the Lobsters, Shrimps, Prawns, and Cray-fish, have a long 

 and well-developed abdomen, the posterior extremity of which 

 forms a powerful natatory organ or caudal fin. This is con- 

 stituted by a greatly-expanded pair of natatory appendages 

 (" swimmerets ") borne upon the last segment but one of 

 the abdomen, between which is placed the last segment or 

 " telson." 



The Macrurous Decapods are unquestionably of a lower 

 type than the Brachyurous Decapods or Crabs ; so that it is 

 no matter of surprise to find that the former, so far as is 

 known, have enjoyed a considerably higher antiquity than the 

 latter. The Brachyura, with two exceptions, are not known 



Fig. 216.Anthrapalcemon Salteri. Carboniferous. (After Salter.) 



in deposits older than the Oolites. The Macrura, on the 

 other hand, were in existence before the close of the Palaeo- 

 zoic period. In the Carboniferous formation we have several 

 forms of Macrurous Crustaceans belonging to the genus An- 

 thrapalcemon (or Antkracopalcemori), of which a species is 

 figured here (fig. 246). In both Europe and North America 

 forms referable to this ancient type have been detected, and 

 one of the most remarkable of these is the Anthrapalcemon 



