STRUCTURE AND APPENDAGES OF TRINUCLEUS 225 



They carry very long, close-set, overlapping, lamellose fringes, 

 which evidently had a branchial function. Some of the 

 lamellae are spiniferous. The exopodites become shorter on 

 the pygidium, and apparently are represented near the end 

 of the series of limbs by the oval plates indicated at <?, figure 

 6. If this interpretation is correct, the posterior exopodites 

 are simple flabella attached to the limbs, as in Apus. 



Both Professors A. E. Verrill and S. I. Smith agree that 

 the characters of the appendages in Trinucleus indicate an 

 animal of burrowing habit, which probably lived in the soft 

 mud of the sea-bottom, much after the fashion of the modern 

 Limulus. In addition to its limuloid form, the absence of 

 eyes seems to favor this assumption. So does the fact that 

 many specimens have been found preserving the cast of the 

 alimentary canal, showing that the animal gorged itself with 

 mud like many other sea-bottom animals. 



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