98 GUIDE TO THE FOSSIL INYEETEBEATE ANIMALS. 



Gallery linibs in front of these are called niaxillipeds, because they 

 assist the mandibles and the two pairs of maxillae in the 

 work of jaws ; and the two pairs in front of all these act as 

 feelers. The fossil Decapoda belong to two Sub-Orders, the 

 Macrura (long-tails) and the Brachyura (short-tails). 

 These represent two grades of structure, the former being 

 the older ; and the most interesting among fossil Decapoda 

 are those that cast light on the evolution of the short-tails 

 from the long-tails, or, as one may put it broadly, the change 

 of lobsters into crabs. 



The British series of fossil decapods is arranged under 

 the time divisions : Jurassic, Cretaceous, and Tertiary ; and 

 within each of these divisions the Macrura precede the 

 Brachyura. True decapods are first found with certainty in 

 Lower Triassic rocks, but these are only represented in the 

 foreign series. 



We shall now take those tribes of Decapoda that are 

 found fossil, in an order corresponding approximately to that 

 of their appearance in the rocks. This order agrees with an 

 arrangement according to grades of structure, the most 

 simple and primitive coming first. 

 Table-case In the foreisjn Trias and in the Lower Lias of Encyland 

 Wall-ease ^® found a long-tailed genus Aeger, in which the first three 

 13a. pairs of thoracic legs bear pincer-claws as in the lobster, but 

 here the third pair is much the largest. For this reason and 

 because of its general form, Aeger is held to be an ancestor 

 of the tribe Stenopidea. It is also found in the Kimmerid- 

 gian lithographic stone of Solenhofen. 



A large prawn, common in the Mediterranean and called 

 Fenaeus, differs from true prawns in having the first three 

 pairs of thoracic legs all much of a size and all, as a rule, 

 with pincer-claws. The tribe Penaeidea of which this is 

 Table-case typical is also supposed to be represented in the Trias. The 

 22. early fossils, however, are rather doubtful, and it is in the 

 Wall-case Solenhofen stone that we first certainly meet with Penaeidea 

 13a. -^^ ^YiQ genera Atrimpos, Acanthochirus, Bylgia, Drohna, Dusa, 

 and others. Fenaeus itself is found in the Senonian rocks 

 of Westphalia. A few examples of the tribe occur in 

 Tertiary strata. 



The true prawns and shrimps, which with their allies 

 form the tribe Caridea, have pincer-claws on the first two 

 pairs of legs, and have the side-plates of the second abdominal 

 segment broadened so as to overlap those of the segments 

 in front and behind. Owing to their comparatively tender 



