Br. H. Woodward — On the Structure of Trilohifes. 79 



the same organs preserved in Crustacea of equal antiquity [e.g. 

 Slimonia acuminata and Eiirypteriis remipes, both Upper Silurian 

 forms). 



We know of no Crustacean having tivo pairs of appendages to each 

 segment ; but it is characteristic of Crustacea to have their append- 

 ages hifid, giving rise to an endopodite and an exopodite, but these 

 are always given off from a common base (basipodite) . 



Having regard to the characters presented by the Trilobita as a 

 group, we should be inclined to place them neai- to (if not actually 

 in) the Isopoda-Normalia. In all this group, the branchia3 are ab- 

 dominal, being placed under the broad and well-developed pygidium, 

 which is not equivalent to the telson of the higher Crustacea, but is 

 composed of several segments soldered together, in fact, representing 

 the true abdomen. It is here, then, we should expect to find the 

 branchiee in the Trilobites placed, and not upon the epimera of the 

 body-segments. 



If any objection should be urged against the organs observed in 

 the specimen in the British Museum being really legs, I would sug- 

 gest that they may be considered good evidence of the pi'esence 

 of these organs, and that they probably represent the apodemata, or 

 infoldings, of the hard external crust to which those oi'gans, or the 

 muscles by which they are moved, were attached. 



The prominence of the hypostome in the Trilobita reminds one 

 even more strongly of the genus Apus than of the Isopods ; and it is 

 quite reasonable to expect, in the Trilobita, a more generalized t^^pe 

 of structure than that which marks the modern (and more specialized) 

 representatives of the class. 



The question will naturally be asked why so many specimens of 

 Trilobites should be found, yet no trace of limbs. It seems reason- 

 able to infer that a large number of these fossil remains are only 

 exiwice, the Crustacea fi-equently casting their shells. The detach- 

 ment of the limbs is also a common occurrence in all the fossil 

 Articulata, especially, where, as in the Crustacea, the proximal joint 

 is extremely constricted, and, in consequence, easily disarticulated. 



Nor need we assume that all the genera of this very extensive 

 family had horny chitinous limbs, seeing that in the modern Isopoda 

 a great diversity exists in these organs. 



The publication of Mr. Billings's discovery appears to me to be of 

 the highest importance to paljBontologists ; and he is entitled to our 

 best thanks ; for his observations are sure to excite further researches 

 upon the Trilobita, and thus will be the means of greatly increasing 

 our knowledge of this interesting group." 



It is not a little singular that the writer of the subjoined article 

 should have met with a specimen of Asaphus broken open and dis- 

 playing the same characters, but, if possible, more clearly than in. 

 Mr. Billings's specimen already noticed. We give the paper as it 

 stands. 



