ARTHROPODA — CRUSTACEA. 
101 
are not developed so constantly or to such an extent. Thus, 
the outline of the body is longer, more of the abdomen can 
be seen from above, its last segment often bears small tail- 
appendages, and in the female the first abdominal segment 
also has traces of a pair of limbs, the gills are more numerous, 
the cavities for the first feelers are not so clearly defined. 
In these characters the Dromiacea approach the Macrura, 
and it has been shown that they more particularly resemble 
the true lobsters. A peculiarity, not derived from the 
lobsters, is that in the Dromiacea the last pair or two pairs 
of thoracic legs are turned on to the back, where, by their 
hooks or claws, they hold a bit of sponge or some such 
object, under which the animal is completely hidden. It is 
to tliis tribe that most of the early fossil Brachyura probably 
belong, although this cannot be determined with certainty, 
because the tender-skinned abdomen is very rarely preserved. 
A starting-] )oint for the tribe is furnished by Frosopon, 
British specimens of which from the Great Oolite are here 
exhibited. First found in Bajocian beds, it persisted to 
Xeocomian times. The carapace, which alone is known, is 
elongate and closely resembles that of Homalodromia, now 
living in the West Indies, and apparently the most primitive 
of recent Dromiidae. Another precursor of this family was 
Fig. 49. — An ancestor of the Crabs, Frotocarcinus longipes of the tribe 
Dromiacea, Great Oolite, Wiltshire. (After Bather.) 
Protocarcimis, found in the Forest Marble, fortunately with 
its limbs. The unique specimen disappeared when its owner 
died ; but a plaster cast is exhibited, and the drawing 
(Fig. 49) made from this shows the primitive nature of the 
carapace, abdomen, and limbs. The English Gault has 
yielded Homolopsis, which appears to lead from Frosopon 
Gallery 
VIII. 
Table-ease 
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