17S The America It (reologist. September, 1895 
cheeks visible. Just such conditions are satisfied in the 
youngest larva of Pttichoparia, Solenopleurn and Liostracus, 
which are the most primitive genera whose protaspis is known. 
The eye-line is present in the later larval and adolescent stages 
of these genera, and persists to the adult condition. In Sao it 
has been pushed forward to the earliest protaspis, and is also 
found in the two known larval stages of Triarthrns. Sao re- 
tains the eye-line throughout life, but in Triarthrus the adult 
has no traces of it, and none of the higher and later genera 
studied has an eye-line at any stage of development. Mat- 
thew has considered this feature as especially characteristic 
of most of the Cambrian genera, and now it is further shown 
to be a character first appearing in the later larval stages of 
certain genera (Pfycftoparia, etc.), next in the larval stages 
(Sao), then disappearing from adult stages (Triarthrus), and 
finally pushed out of the ontogeny altogether (Acidaspis, Dal- 
maiiifes, etc.). The eyes are visible on the margin of the dor- 
sal shield after the paraprotaspis stage, later than the eye-line 
in Ptychoparia, Solenopleura, Liosfracus, Sao and Triarthrus; 
"but in the other genera through acceleration they are present 
in all the protaspis stages, and persist to the mature, or ephe- 
bic, condition, moving in from the margin to near the sides of 
the glabella. 
The changes in the glabella are equally important and in- 
teresting. Throughout the larval stages, the axis of the ceph- 
alon is five-segmented or annulated, indicating the presence of 
as many paired appendages on the ventral side. In its sim- 
plest and most primitive state, it expands in front, Joining and 
forming the anterior margin of the head (larval Ptiichoparia, 
Sao). During later growth it becomes rounded in front and 
terminates within the margin. In higher genera through ac- 
celeration it is rounded and well-defined in front even in the 
earliest larval stages and often ends within the margin (lar- 
val Triarthrus, Acidaspis). From these common types of sim- 
ple, pentamerous glabellse, all the diverse forms among adult 
individuals of various genera have been derived, through 
changes aifecting any or all of the lobes. The modifications 
usually take place in the anterior lobes first, and gradually in- 
volve the others, though rarely disturbing the neck segment 
which is the most persistent of all. Six lobes are occasion- 
