LARVAL STAGES OF TRILOBITES 179 



the primary larval characters of the trilobites, and therefore 

 only the earliest stages are considered. Under the genera 

 just mentioned the writer has endeavored to show that as 

 yet their ontogeny cannot be traced as far back as the stage 

 which has been defined as the protaspis. Therefore any 

 general notions of first larval forms must at present be based 

 on the genera Solenopleura, Liostracus, Ptychoparia, Sao, 

 Triarthrus, Acidaspis, Proetus, and Dalmanites. 



Analysis of Variations in Trilobite Larvce. 



After taking a general survey of the earliest known larval 

 stages of trilobites figured on Plates III and IV, it is evident 

 that an accurate and detailed description of any one would 

 not apply to any other except in certain broad characters. 

 To formulate a definition of the protaspis applicable to all, 

 as has been done previously (pp. 169 and 170), it is necessary 

 to neglect or eliminate some rather striking characters which 

 should now be mentioned. A few, features thus omitted are 

 considered as very primitive larval characters, while others 

 are modifications introduced in higher or later genera through 

 the operation of the law of earlier inheritance. 



From the best evidence now obtainable, the eyes have 

 migrated from the ventral side, first forward to the margin 

 and then backward over the cephalon to their adult position, 

 thus agreeing with Bernard's conclusions. 12 Therefore the 

 most primitive larvae should present no evidence of eyes on 

 the dorsal shield, and naturally there would be no free- 

 cheeks visible. Just such conditions are satisfied in the 

 youngest larva of Ptychoparia, Solenopleura, 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 Triarthrus. Sao retains the eye -line throughout life, but 

 in Triarthrus the adult has no traces of it, and none of the 



