ON CERTAIN VERMIFORM FOSSILS. 33 



the middle line, and in one or two instances this elevated portion 

 has occurred broken up into a series of irregular nodules (e, c), as 

 if the animal had at each step in its progress thrust the head 

 upwards, bulging out the walls of the track. The lateral por- 

 tions, in these cases, are likewise raised up into irregular ridges 

 at each nodule. 



Burmeister, indeed, in his work on the " Organization of 

 Trilobites,"* expresses an opinion that their habits, like their 

 structure, resembled those of the Pliyllopoda^ a tribe of the 

 Entomostraca, and that they " moved only by swimming in an 

 inverted position close beneath the surface of the water, and did 

 not creep about at the bottom as Mr. Kloden supposed." 

 Though their habits may have been similar to those of the Pliyl- 

 lopoda^ there does not seem any good reason for asserting that 

 there was no deviation in this respect. Indeed, the organization 

 of the two groups differs in so many particulars that some varia- 

 tion in their modes of life might naturally be looked for. The 

 trilobites may have occasionally swum at the surface as supposed, 

 and also have burrowed in the mud or sand at the bottom of the 

 water, or on the beach. Season, too, may have modified their 

 habits in these respects. 



, I have now, before concluding, only to express my obligations 

 to Mr. Wood for all the trouble he has taken to furnish me with 

 information on the subject of these remarks, and likewise for 

 the great exertions he has made to supply specimens for illus- 

 tration. My acknowledgments are also due to Mr. Howse for 

 similar assistance. 



EXPLANATION OF PLATES. 



PLATE III. 



Fig. 1. Broad, grooved track of Suhator arenarius : a, groove ; 5, one of the 

 extremities ; c, inconspicuous, arched ridgef, which occasionally occur. 

 Fig. 2. a, nodulous track of ditto ; 6, ditto, ditto made in confinement. 



* Published by the Ray Society. 

 VOL. IV. PT. I. E 



