140 STUDIES IN EVOLUTION 



It has been recognized that variations in the position of 

 the eyes, the relative size of the free- and fixed-cheeks, and 

 the degree of specialization of the glabella have a definite 

 order in the ontogeny of any trilobite, and also that these 

 characters have a greater taxonomic value than many others. 

 Applying these principles in arranging the families which 

 come under the Opisthoparia, we have the sequence as indi- 

 cated above, beginning with the Conocoryphidse and followed 

 by the Olenidaj, Asaphidse, Proetidse, Bronteidse, Lichadidse, 

 and Acidaspidae, in regular progression. See Plate II. 

 figures 14-23. 



Family IV. ConocoeyphidjE Angelin. 



Free-cheeks very narrow, forming the lateral margins of the 

 cephalon, and bearing the genal spines. Fixed-cheeks large, 

 usually traversed by an eye-line extending from near the ante- 

 rior end of the glabella. Facial sutures running from just within 

 the genal angles, curving forward, and cutting the anterior 

 lateral margins of the cephalon. Eyes rudimentary or absent. 

 Thorax with from fourteen to seventeen segments. Pygidium 

 small and of few segments. Cambrian. 



Including the genera and sub-genera Conocoryphe Corda (= Co- 

 noc&phalites Barrande), Aneucanthus Angelin, Atops Emmons, 

 Avalonia Walcott, Bailiella Matthew (= Salteria Waleott and 

 Erinnys Salter), Bathynotus Hall, Carausia Hicks, Carmon 

 Barrande, Ctenocephalus Corda, Dictyocephalites Bergeron, Eryx 

 Angelin, Harttia Walcott, and Toxotis Wallerius. 



The genera coming under this family present a number of 

 very primitive characters such as are shown only in the larval 

 stages of higher forms. The free-cheeks are narrow and 

 marginal, and can be compared with those in the nepionic 

 stages of Sao and Ptychoparia. The eyes have not been 

 detected, but the presence of an eye-line suggests their pos- 

 sible existence. The variations of the glabella are very 

 marked, and are as great as those which in higher forms 

 attain some importance as family characteristics. In Toxotis, 

 Carausia, and Aneucanthus the glabella expands in front, 



