. BRITISH FOSSILS. ' 35 



The palp of the mandible of Nebalia (Plate XVI. fig. 10) is, in its 

 : proportional size and form, not unlike that of Pterygotus anglicus, 

 but this is the sole resemblance of importance which I can detect 

 between the Branchiopoda and the Pterygoti. 



While the relations of the Pterygoti with the great majority of 

 other Crustacea, are such as, in my mind, fully to justify their ordinal 

 separation, there is one small and extinct group, having a similar 

 geological range, with which they are undoubtedly closely connected. 

 This is the genus Eurypterus of Harlan, of which many species have 

 now been brought to light. Putting together the descriptions of 

 Harlan, Eichwald, Roemer,* and others, with the results of a personal 

 inspection of some species recently described by Mr. Salter, I can 

 only arrive at the conclusion that, in its general form and structure,, 

 Eurypterus very closely resembled Pterygotus, differing from it 

 however as Sapphirina differs from Corycceus, viz., in having the eyes 

 submedian instead of marginal. The ectognaths of Eurypterus 

 appear to have been constructed upon precisely the same plan as those 

 of Pterygotus, and its thoracic and abdominal members would seem to 

 have been equally undeveloped.! Eichwald describes twelve segments 

 behind the carapace (though only eleven are represented in his figure). 

 Two pairs of slender jointed appendages are figured both by him| 

 and by Harlan in front of the ectognaths ; but no chelate antennae 

 are represented, and the connexions of the jointed appendages are 

 not made out. The characteristic sculpture is well exhibited by 

 Eurypterus. Even if the chelate antennae are really absent in 

 Eurypterus, it must be remembered that these are organs particu- 

 larly liable to variation among even closely allied genera of 

 Copepoda, 



Leaving the determination of the precise differences between the 

 Pterygotus and Eurypterua to future investigators, their resem- 

 blances to one another, and their common differences from all other 



* Ferd. Roemer. Ueber ein bisher nicht beschriebenes Exemplar von Eurypterus aus 

 Devonischen schichten des Staates New York in Nord-Amerika. Dunker und Von Meyer's 

 Paleontographica, b. 1, 1851, p. 190. 



f Eichwald's representation of the under surface of the head is terribly diagrammatic. 

 If it be a correct representation of an actual object, it affords evidence of only three pairs of 

 appendages. His " two large, almost semilunar, lateral parts " appear to me to be the 

 basal joints of the ectognaths, while the middle ovate "under lip" has just the position 

 and general form of the metastoma. Add to these the triangular " upper lip," and the 

 resemblance to Pterygotus becomes not a little striking. 



J Roemer states that his Eurypterus also had twelve free segments behind the carapace. 

 The eyes borne by the latter were uniform and not facetted ; only two appendages, both 

 on the left side, remained. The posterior had the general structure of the ectognathary 

 limb of Pterygotus. The anterior is slender, four-jointed, and perhaps terminated in a 

 chela. 



c 2 



