36 BRITISH FOSSILS. 



Crustacea, are sufficiently important to justify their association into 

 a distinct order the " Eurypterida" which may be thus denned : 

 Crustacea with numerous free thoracico-abdominal segments, the 

 penultimate, and probably all the rest of which, are devoid of appen- 

 dages ; with the anterior somites united into a carapace, bearing a 

 pair of large marginal or subcentral eyes ; with a very large epistoma 

 and metastoma ; with three, or four pair of moveable cephalic ap- 

 pendages, the posterior of which form great swimming feet, and 

 with the integument characteristically sculptured. 



Of the mode of life of these extinct Crustacea we know nothing, 

 but we may conjecture that it was their habit sometimes to creep 

 along the bottom of the waters which they inhabited, like theLimulus 

 of the present day, sometimes to propel themselves by the rapid 

 flexion of their great swimming feet, aided perhaps by the sudden 

 extension of their free segments. 



No existing or extinct crustacean has so massive a body as 

 Pterygotus, some species of which there is every reason to believe 

 attained a length of at least five feet ; but mass, in an active animal, 

 involves large muscles, and these require solid points d'appui. 

 Hence we may conclude that the integument of the Pterygotus, thin 

 and fragile as are its remains, possessed a great amount of firmness 

 in the recent state. 



That the singular thinness of the fossil test, constituting a mere 

 papyraceous film, is not inconsistent with great solidity in the recent 

 condition, is shown by the existing Limulus, whose leathery integu- 

 mentis thick and hard enough to give firm attachment to very large 

 and powerful muscles, and yet contains so little calcareous matter, 

 that when dried and subjected to such pressure and decomposing 

 influences as have operated upon the Pterygoti, it would probably 

 be hardly more bulky. 



It is not without interest to find that the skeleton was as perishable 

 in the Crustacea as in the Vertebrata of the period in which the 

 Pterygoti flourished. 



Note, p. 28. So long ago as 1840, Milne Edwards indicated the relations of Eurypterus 

 -with the Copepoda in the following -words : " The fossil Crustacea of -which M. Dekay 

 has formed the genus Eurypterus appear to have much analogy with Pontia (Pontella') 

 and Cyclops, and also seem in gome respects to form a link between these animals and the 

 Isopoda" Hist. Nat. des Crustaces, t. iii. p. 422. 



