128 STUDIES IN EVOLUTION 



On the other hand, the apparently unlimited multiplication 

 of thoracic and especially of abdominal segments in some 

 genera is also to be considered as a primitive character 

 expressive of an annelidan style of growth. Genera like 

 Asaphus, Phacops, etc., having a constant number of thoracic 

 segments accompanied by other characters of a high order, 

 undoubtedly represent the normal trilobite type. 



These analyses and correlations clearly show that there are 

 characters appearing in the adults of later and higher genera, 

 which successively make their appearance in the protaspis 

 stage, sometimes to the exclusion or modification of struc- 

 tures present in the most primitive larvae. Thus the larvae 

 of Dalmanites or Proetus, with their prominent eyes and 

 glabella distinctly terminated and rounded in front, have 

 characters which do not appear in the larval stages of ancient 

 genera, but which may come in their adult stages. Evi- 

 dently such modifications have been acquired by the action 

 of the law of earlier inheritance, or tachygenesis. 



In a classification of trilobites, for the purpose of illustrat- 

 ing the principles here enunciated, the ontogenies of Sao and 

 Dalmanites, Plate II, figures 1-8, are selected. Sao belongs 

 to the ancient family Olenidse of the order Opisthoparia, and 

 naturally may be expected to furnish very clear evidence as 

 to the relations of many lower and older genera. Dalmanites, 

 also, with its simple head structure, will give similar data 

 regarding the Proparia. 



The early protaspis stage of Sao, Plate II, figure 1, has 

 no dorsal development of the free-cheeks, and with the elon- 

 gate form of the cephalic portion may be compared with the 

 cephala of Agnostus and Microdiscus, and therefore correlates 

 with the Hypoparia. The cephalon, at a later period of 

 development, when the animal has two free thoracic seg- 

 ments, Plate II, figure 2, shows the narrow marginal free- 

 cheeks and distinct eye-lines. Here the resemblance to the 

 cephala of Atops and Conocoryptie, Plate II, figures 14, 15, 

 is very marked, and indicates that the Conocoryphidas are 

 genetically the first family of the Opisthoparia. When Sao 



