Observations on Trilobites. 18) 



voyage, will show the structure and mechanism of these lateral 

 lobes, better than the most detailed description. 



Admitting then this opinion, there would be little occasion 

 for feet, or they might be merely rudimentary. This is in 

 conformity to the law of organization, by which the loss of 

 certain parts is supplied by an extra development of others to 

 perform the same functions. Thus, not to go out of the class 

 of the Crustacea, Hipp a and its allied genera have minute 

 and almost abortive claws, and no pincers at their extremities. 

 To compensate for this, the mouth with its large horny jaws, 

 is so much enlarged, as to serve as an organ of prehension. 



The question, however, still recurs, to what genus of the 

 crustaceous animals, are the Trilobites nearest allied ? In 

 other words, what living animal most resembles this extinct 

 being? Our observations have led us to adopt the opinion, 

 that the Isotelus, and by analogy other trilobites, bear a strik- 

 ing affinity to the Limulus polyphemus, or common " Horse- 

 foot" of our shores.* 



1 . The Limulus itself has been found, though rarely, in a 

 fossil state. By comparing these with the living Limulus, M. 

 Desmarest, remarked that the inferior (posterior?) angle of the 

 buckler was more obtuse in the fossil. The Isotelus also has 

 these angles more obtuse. The same distinguished observer 

 states, that the tail of the fossil species, is divided by longitudi- 



* This large and common species has been much neglected by on: 

 anatomists. 



The following are the species as near as they can be determined from the 

 confused accounts in the different systems. 



1 $ Limulus polyphemus. Latreille, Bosc. ) -.j -.-j, q. : 



( Polyphemus occidentalis. Lamarck. £ 



2. L. mollucanus. Latr. P. gigas Lam. Molluca Islands. 



3. L. hcterodactylus. Lat. China. 



L. ryclops. Bosc. doubtful. L. albus. Bogc. The cast-off shell of a 

 young Limulus ? 



L. Soiccrbii. Leach, probably the polyphemus not full}' grown. 



