102 GUIDE TO THE FOSSIL INVERTEBRATE ANIMALS. 
Gallery 
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towards the family Iloinolidae. Dromwpm, funnel in the 
Upper Chalk of Denmark, is an obvious representative of 
the Dromiidae, and Lromilites (Fig. 4G, J), of which there is 
shown a good series from the Loudon Clay, is scarcely 
different from the modern Droviiu. 
Nearer to the true crabs is the trihe Oxystomata (the 
Sand-crabs), which owes its technical name to the fact that 
the month-lrame is narrowed in front and projects forward 
between the eyes. In most crabs the month-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 example being 
Mithracites vcctensis, from the Lower Greensand of Atherfield. 
In the English Gault and Upper Greensand are found Palaeo- 
corystes (Fig. 44, 2), Eiicorystcs, Nccrocarcinvs, OrWu^sis, and 
Trachynotns. Tlie precise relations of these to modern families 
are doubtful. The family Ilaninidae, found first in the 
Cenomanian Chalk, and not rare in Tertiary rocks, is illus- 
trated by foreign si)ecimens from the Eocene of Kressenberg 
and Scinde and the IMiocene of Gironde. Other families, 
represented by genera still living, also appear in Tertiary 
times ; Calaj)pci, for instance, is represented in the Lritisli 
series by I'ragments 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 — 
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 LithoiJhylcuc, a doubtful 
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 crahs belong to the tribe Cyclo- 
metopa, in which the carapace is, as a rule, broader than 
lung, with the front curved and not produced into a rostrum. 
'With the exception of the river-crabs, all modern forms of 
