1/8 ANNULOSA. 



The order Decapoda 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 : 



TRIBE I. MACRURA. 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 " tel- 

 son." 



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 vastly higher antiquity than the latter. 

 The Brachyura are not known in deposits older than the 

 Oolites. The Macrura, on the other hand, were in existence 

 before the close of the Palaeozoic period. In the Carboni- 

 ferous formation we have several forms of Prawn-like Crusta- 

 ceans belonging to the genus Anthrapalcemon (or Anthracopa- 

 fomori), of which a species is figured below (fig. 121). In 



Fig. iz-L.Anthrapalcemoti Salteri, Carboniferous. (After Salter.) 



the Permian Rocks no Macrurous Decapods are known to 

 occur. In the Trias, examples of the genera Galatea and 

 Litogaster have been detected, and other forms have been 

 alleged to occur. In the Jurassic and Cretaceous strata 

 " Long-tailed " Decapods are extremely abundant, and are 

 often beautifully preserved. Amongst the more remarkable 

 of the Jurassic genera may be mentioned Eryon (fig. 122), 



