58 BRITISH FOSSIL CRUSTACEA. 
Fic. 10. Detached organs from the head of Pterygotus bilobus (var. a), figured in PI. X, fig. 3. 
1, 1. The chelate antenne. 
2,2. The basal joints of the 1st pair of endognaths. 
Osos A a) end), A 
4, 4. 5 SOL Css op 
5, 5. The ectognaths, or maxillipeds. 
m. The metastoma, or post-oral plate. 
-e, e, e. Portions of the filiform appendages of the endognaths. 
The letters to the joints of the ectognath correspond with the articuli as enumerated in Part I, p. 6. 
(The organs are here represented of the natural size.) 
The first joint is 3 lines in length by 1 line at its proximal and 2 lines at its distal 
end in breadth; the second and third are each 6 lines in length and 2 lines in breadth; 
the fourth is 6 lines to the base of the articulation of the moveable ramus, the rami being 
7 lines in length ; the fifth joint is the moveable ramus already described. 
The antennze appear to have been epistomial organs like the inner antennee of Limulus 
(see Part I, Pl. IX, fig. 1,2); we have no evidence of their having been furnished with 
a mandible at their proximal end, as is the case with the succeeding four pairs of oral 
appendages, the endognaths and ectognaths. We shall, therefore, continue to consider 
their point of attachment to be anterior to the mouth, as represented in Part I, Pl. VIII, 
nee 1, 2. 
The Endognaths (which may be considered as equivalent to the mandibles and the first 
and second maxilla of the higher Crustacea) are the next three pairs of oral appendages. 
