456 Mr. A. Hancock on Vermiform Fossils, 



this form. The P. gemmulifera has the pygidium five lines wide ; 

 so has Griffithides calcaratus ; and two or three other species arc 

 described to be about the same size. And, moreover, the large 

 cephalic shield, with its anterior or head-tubercle (glabella) and 

 projecting " cover of the eyes," appears well calculated to plough 

 its way beneath the surface of the sandy or muddy beach. And 

 it is worthy of remark, that in some of the tracks the central 

 portion is considerably elevated, forming the upper surface or 

 roof into three areas — a central elevated portion (PL XIX. fig.2 a) 

 and two comparatively depressed lateral portions (c, c). The 

 former, which appears to correspond to the glabella, is grooved 

 (b) along the middle line ; and in one or two instances this ele- 

 vated portion has occurred broken up into a series of irregular 

 nodules (e, e), as if the animal had, at each step in its progress, 

 thrust the head upwards, bulging out the walls of the track. 

 The lateral portions in these cases are likewise raised up into 

 irregular ridges at each nodule. 



Burmeister, indeed, in his work on the f Organization of Tri- 

 lobites*/ expresses an opinion that their habits, like their struc- 

 ture, resembled those of the Phyllopoda (a tribe of the Ento- 

 mostraca), and that they " moved only by swimming in an in- 

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

 not creep about at the bottom, as Kloden supposed." Though 

 their habits may have been similar to those of the Phyllopoda, 

 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 variation 

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

 lobites 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 THE PLATES. 

 Plate XIV. 



Fig. 1. Broad, grooved track of Sidcator arenarius : a, groove; b, one of 

 the extremities ; c, inconspicuous arched ridges, which occasion- 

 ally occur. 



Fig. 2. a, Nodulous track of ditto ; b, ditto, ditto, made in confinement. 



* Published by the Ray Society. 



