No. 425.] THE TREMATASPIDE. 393 
and specialization of the anterior body segments into a head 
region is definitely begun and completed. It is only when this 
group is viewed as a whole that we see these momentous 
structural advances in their true perspective, and can follow 
the endlessly varied theme that leads steadily and consistently 
onward toward the completion of the most complex organic 
structure that has ever been produced, the vertebrate head. 
DARTMOUTH COLLEGE, 
January, 1902. 
