566 CRUSTACEA. 



during their eggs, it is conceivable that by their agency a curious 

 and puzzling intermixture of land-shells with marine types of Mol- 

 luscs might be effected. 



With regard to the distribution of the Anomura in time, our 

 knowledge is at present very imperfect. If the group of Decapods 

 represented by the living Sponge-crabs (Dromia) be referred to the 

 Brachyura, then it is doubtful if any remains of the Anomura have 

 been found in any Mesozoic deposit, while Palaeozoic forms are 



Fig. 428. — Hermit-crab (Cocnobita) in its borrowed shell. After Morse. 



wholly unknown. The claws of a form resembling Galathea have 

 been recorded as occurring in the Faxoe Chalk ; but with this ex- 

 ception the earliest unquestionable remains of Anomurous Decapods 

 are referable to the genus Bagurus, and are found in the Eocene 

 deposits. 



Tribe C. Brachyura. — The " short-tailed " Decapods, or Crabs, 

 are distinguished from the two preceding tribes by the rudimentary 

 condition of the abdomen, which is very short, and is tucked up 

 beneath the cephalothorax, the latter being disproportionately large. 

 There is no caudal fin, and there are from one or two (males) to four 

 (females) pairs of abdominal appendages, which are employed by 

 the females in carrying the ova. The Crabs are mostly furnished 

 with ambulatory limbs, and are rarely formed for swimming, most 

 of them being littoral in their habits, and some even living inland. 



With regard to their geological history, it is very doubtful if any 

 genuine representatives of the Brachyura have been hitherto de- 

 tected in the Palaeozoic series. The Gitocrangon gi-anulatus of the 

 Devonian rocks, the Brachypyge carbonis of the Carboniferous, and 

 the Hemitrochiscus fia?-adoxus of the Permian deposits, which have 

 been supposed to be ancient types of the Brachyura, are all more 

 or less problematical in nature. Even in the Jurassic rocks the 



