98 GUIDE TO THE EOSSIL INVERTEBEATE ANIMALS. 
Gallery 
VIII. 
Table-case 
22 . 
Wall-case 
13a. 
Table-case 
22 . 
Wall-case 
13A. 
limbs in front of these are called maxillipeds, because they 
assist the mandibles and the two pairs of maxillae in the 
work of jaws ; and the two pairs in front of all these act as 
feelers. The fossil Decapoda belong to two Sul)-Orders, the 
Macrura (long-tails) and the Brachyura (short -tails). 
These represent two grades of structure, the former being 
the older ; and the most interesting among fossil Decapoda 
are those that cast light on the evolution of the short-tails 
from the long-tails, or, as one may put it broadly, the change 
of lobsters into crabs. 
The British series of fossil decapods is arranged under 
the time divisions : Jurassic, Cretaceous, and Tertiary ; and 
within each of these divisions the Macrura are placed before the 
Brachyura. True decapods arc first found with certainty in 
Lower Triassic rocks, but these are only represented in the 
foreign series. 
We shall now take those tribes of Decapoda that are 
found fossil, in an order corresponding approximately to that 
of their appearance in the rocks. This order agrees witli an 
arrangement according to grades of structure, the most 
simple and primitive coming first. 
In the foreign Trias and in the Lower Lias of England 
is found a long-tailed genus Aeger, in which the first tliree 
pairs of thoracic legs bear pincer-claws as in the lobster, but 
here the third pair is much the largest. Eor this reason and 
because of its general form, Aeger is held to be an ancestor 
of the tribe Stenopidea. It is also found in the Kimmerid- 
gian lithographic stone of Solenhofen. 
A large prawn, common in the ^Mediterranean and called 
Penaeus, differs from true prawns in having the first three 
pairs of thoracic legs all much of a size and all provided 
with pincer-claws. The tribe Penaeidea, of which this is 
typical, is also supposed to be represented in the Trias. The 
early fossils, however, are rather doubtful, and it is in the 
Solenhofen stone that w’e first certainly meet with Penaeidea 
in the genera Atrimgios, AcantlwcMrus, Bylgia, Drohna, Dusa, 
and others. Penaeus itself is found in the Senonian rocks 
of Westphalia. A few examples of the tribe occur in 
Tertiary strata. 
The true prawns and shrimps, which with their allies 
form the tribe Caridea, have pincer-claws on the first two 
pairs of legs, and have the side-plates of the second abdominal 
segment broadened so as to overlap those of the segments 
in front and behind. Owing to their comparatively tender 
