BRITISH FOSSILS. 19 



linear impression first described, the source of the latter will be 

 obvious. 



The base of this great limb is attached to the hinder part of the 

 carapace ; it therefore, without doubt, lay behind the mouth, and 

 as it is the outermost pair of buccal appendages, it will be useful to 

 call it the " ectognath," a term more convenient than " maxilla," 

 " maxillipede," or " swimming-foot/' as it involves neither a morpho- 

 logical nor a physiological hypothesis. 



The organ which caused the shorter linear impression, lying- 

 internal to and in front of, that of the cutting edge of the ectognath 

 is, I doubt not, similar to that which lies detached in two speci- 

 mens, and is figured in the woodcut, p. 40. It is unfortunately 

 impossible in either of these examples to trace, as distinctly as might 

 be wished, the precise form of this appendage, but enough is visible 

 to satisfy me that this species was provided with one, if not two, 

 organs on each side similar in general structure to the appendage (6) 

 of P. anglicus, and quite competent to produce the impression in 

 question. 



From its general character I entertain very little doubt that the 

 appendage (6) is the homologue of the mandible of other Crustacea, 

 but as there is some reason to suspect the existence of a third buccal 

 appendage, and as it is impossible as yet, supposing this third jaw 

 to exist, to say whether any given appendage similar to 6. was first 

 or second in order, it will be better to term an oral limb in front of 

 the ectognath, an "endognath." The specific name of" mandible" 

 must be assigned to whichever of these endognaths further research 

 proves to be anterior. 



The position and general nature of two of the three pairs of 

 appendages of Pterygotus having thus been determined, the third 

 presents no difficulty. In the specimens of P. bilobus, represented 

 in Plate I. figs. 1, 3, indeed, the pair of long anterior appen- 

 dages are so much distorted and damaged that their structure 

 cannot be satisfactorily made out. There is abundant evidence, 

 however, to show that they were similar to the detached chelae 

 figured in Plate I. fig. 6, and these are obviously identical with 

 the pincer-like appendage (a) of P. anglicus. 



The epistoma has already been recognized in the plate lying in 

 front of the head, in Plate I. fig. 1 ; it remains only to discover the 

 representative of the metastoma, and the position which it occupied 

 in the organism. The form and sculpture of the part represented 

 in Plate I. figs. 10, 10 a, testify that it answers to the organ sought, 

 and a careful examination of most well preserved carapaces of 



B 2 



