180 STUDIES IN EVOLUTION 



higher and hiter genera studied has an eye-line at any stage 

 of development. Matthew 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 (Ptyclioparia, etc.), 

 next in the larval stages (Sao)^ then disappearing from adult 

 stages (^Triarthrus\ and finally pushed out of the ontogeny 

 altogether (^Acidaspis, Dalmanites, etc.). The eyes are visi- 

 ble on the margin of the dorsal shield after the paraprotaspis 

 stage, later than the eye-line in Ptychoparia, Soh)iopIe7ira, 

 Liostracus, Sao, and Triarthrus ; but in the other genera 

 througli acceleration they are present in all the protaspis 

 stages, and persist to the mature, or ephebic, condition, 

 moving in from the margin to near the sides of the 

 glabella. 



The changes in the glabella are equally important and 

 interesting. Throughout the larval stages the axis of the 

 cephalon is five-segmented or annulated, indicating the pres- 

 ence of as many paired appendages on the ventral side. In 

 its simplest and most primitive state it expands in front, 

 joining and forming the anterior margin of the cephalon (larval 

 Ptychoparla, Sao). During later growth it becomes rounded 

 in front and terminates within the margin. In higher genera 

 through acceleration it is rounded and well-defined in front 

 even in the earliest larval stages, and often ends within the 

 margin (larval Triarthrus, Acidaspis). From these common 

 types of simple, pentamerous glabellse, all the diverse forms 

 among adult individuals of various genera have been derived, 

 through changes affecting any or all of the lobes. The 

 modifications usually take place in the anterior lobes first, 

 and gradually involve the others, though rarely disturbing 

 the neck segment which is the most persistent of all. Six 

 lobes are occasionally found in the glabellse of some species. 

 They do not indicate an additional pair of limbs, for the 

 extra lobe is produced (a) by division of the anterior lobe 

 through the greater or less extent of the eye-line across the 

 axis, as in Olenellus, Paradoxides and Oyygia ; or (5) by the 



