THORACOSTKACA OR PODOPHTHALMATA. 



565 



tribe. Among the more characteristic Cretaceous genera may be 

 mentioned Enofiloclytia, Hofiloparia, Meyeria, Palifiurus, and Scyl- 

 larus. Very abundant also in parts of the Cretaceous series are 

 the great chelae of burrowing Crabs belonging to the genus Cal- 

 lianassa, a type which appeared in the Jurassic rocks, and which 

 survives at the present day. In the Tertiary rocks, finally, the 

 remains of Macrura are 

 comparatively scanty, and 

 present few points of special 

 interest. The recent genus 

 Homarus, including the 

 common Lobster, appears 

 in the Oligocene deposits, 

 and in the fresh-water Ter- 

 tiary deposits of North 

 America are found the re- 

 mains of Cray-fishes (Asta- 

 cus) essentially similar to 

 existing forms. 



Tribe B. Anomura. — 

 The Decapods which be- 

 long to this tribe are dis- 

 tinguished by the condition 

 of the abdomen, which is 

 neither so well developed 

 as in the Macrura, nor so 

 rudimentary as in Crabs. 

 The last pair of thora- 

 cic limbs are reduced in 

 size. Further, the abdo- 

 men does not terminate posteriorly in 

 Macrura. 



The division Anomura must be regarded as an artificial assem- 

 blage, composed of modified forms of both the Macrura and 

 Brachyura. It is, in fact, impossible to draw a rigid line between 

 the Anomura and the Brachyura — the two tribes being connected 

 by transitional forms, which might with equal propriety be placed in 

 either. The most characteristic forms of the Anomura are the 

 "Hermit-crabs" (Bagurus, Coznobita, &c), and the "Plated 

 Lobsters" (Galathed). The Hermit-crabs are interesting on ac- 

 count of their habit of protecting the soft abdomen within the 

 empty shell of some Mollusc (fig. 428). Many of the Hermits, 

 such as the Coz?iobit<£ of the tropics, are terrestrial in their habits, 

 and commonly employ the shells of snails for their borrowed dwell- 

 ing. As the Coenobite migrate to the sea for the purpose of pro- 



Fig. 427. 



Jurassic rocks. 



-Eryon arctiformis, 

 (Solenhofen Slates.) 



caudal fin, as in the 



