102 GUIDE TO THE FOSSIL INVERTEBEATE ANIMALS. 



Gallery Upper Chalk of Denmark, is an obvious representative of 

 the Dromiidae, and Dromilites (Fig. 46, 1), of which there is 

 shown a good series from the London Clay, is scarcely 

 different from the modern Dromia. 



N'earer to the true crabs is the tribe Oxystomata (the 

 Sand-crabs), which owes its technical name to the fact that 

 the mouth-frame is narrowed in front and projects forward 

 between the eyes. In most crabs the mouth-frame is square, 

 and the channels that carry the outward stream of water 

 from the gills open at its two front corners ; here, however, 

 they are carried forward to the front of the head, and closed 

 below by plates connected with the first maxillipeds. The 

 reason for these changes is, that the Sand-crabs bury them- 

 selves in sand, leaving only the eyes exposed. This tribe 

 appears later than the Dromiacea, its earliest representative 



Table-case being Mithracites vcctensis, from the Lower Greensand of 

 Atherfield. In the English Gault and Upper Greensand are 

 found Palaeocorystes (Fig. 44, 2), Eucorystes, Necrocarcinus, 

 Orithopsis, and Trachynotus. The precise relations of these 

 to modern families are doubtful. The family Eaninidae, 

 found first in the Cenomanian Chalk, and not rare in 

 Tertiary rocks, is represented in the foreign series by Eocene 

 specimens from Kressenberg and Scinde. Other families, 

 represented by genera still living, also appear in Tertiary 



Table-ease times ; Calappa, for instance, is represented in the British 

 series by fragments of Eocene, Oligocene, and Pliocene age. 



The tribe Oxyrhyncha^ characterised by a triangular 

 carapace, with the apex produced in front as a rostrum, is 

 not common in the fossil state. The common genus Maia — 



Table-ease the spider-crab — is, however, represented by specimens from 

 the Coralline Crag of Suffolk, where it must have lived as 

 nowadays, covering itself with masses of bryozoans and sea- 

 weed. 



The crabs of the tribe Catometopa have a squarish 

 carapace with front strongly bent downwards. Their earliest 

 representative is the Upper Cretaceous Lithopliylax, a doubtful 

 Table-case and rare form not shown here. In Eocene rocks they are 

 less rare, and here is to be seen the original of Goniocypoda 

 Edwardd from Hampshire. 



The majority of fossil crabs belong to the tribe Cyclo- 

 metopa, in which the carapace is, as a rule, broader than 

 long, with the front curved and not produced into a rostrum. 

 With the exception of the river-crabs, all modern forms of 

 this tribe live in the sea. Among the oldest genera are 



