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BRITISH FOSSIL CRUSTACEA. 



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Fig. 10. Detached organs from the head of Pterygotus bilobus (var. a), figured in PL X, fig. 3. 



1, 1. The chelate antennae. 



2, 2. The basal joints of the 1st pair of endognaths. 

 o, o. „ ,, 2nd „ „ 



4, 4. ,, j, ora ,, ,, 



5, 5. The ectognaths, or maxillipeds. 

 m. The metastoma, or post-oral plate. 



e, e, e. Tortious 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 antennas appear to have been epistotnial organs like the inner antennae 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, PI. VIII, 

 fig. 1, 2. 



The Endognaths (which may be considered as equivalent to the mandibles and the first 

 and second maxillae of the higher Crustacea) are the next three pairs of oral appendages. 



