SUBCLASS I 



TRILOBITA 



713 



typifying tlie faniily Oleiiidae, govern its position, which accordiiigly woiild be after 

 the Coiiocoryijliidae. 



Ditferences in tlie position of the eyes, the relative size of the free and fixed cheeks, 

 and the degree of specialisation of the glabella have a definite order in the ontogeny of 

 any Trilobite, and furnish characters of taxonomic value in arranging the families 

 placed nnder the Opisthoparia (see Fig. 1364). 



Family 1. Oonocoryphidae Angelin. 



Opisthoparia with free cheeks very narrow, forming the lateral margins of the 

 cephalon, and hearing the genal spines. Eye-lines are present, hut neither ocelli nor 

 Compound eyes. Thorax ivith from fourteen to seventeen segments. Pygidium small. 

 Cambrian. 



The genera comprised under this family present a number of very primitive 

 characters, such as are displayed only in the larval stages of higher forms, The free 

 cheeks are narrow and marginal, and may be compared with those in the nepionic 

 stages of Sao and Ptychoparia. Eyes have not been detected, but the presence of an 

 eye-line suggests their possible existence. The variations in the glabella are very 

 marked, and are as great as those which in higher forms attain some importance as 

 family characteristics. 



So far as known, all the larval forms in the other families of the Opisthoparia 

 agree in having the narrow marginal free 

 cheeks, bearing the genal angles, The 

 eye-line is present in most of the adult 

 Olenidae, and in the early stages of all so 

 far as known, so that the general average 

 of characters in the Oonocoryphidae re- 

 presents the main larval features through- 

 out the other families. 



Conocoryphe Oorda (Figs. 1364, B; 

 1365, 1366). The glabella is convex, 

 tapering toward the front. Anterior 

 border of cephalon marked by a broad 

 and deep furrow. Oambrian ; Europe and 

 North America. 



Ctenocephalus Oorda. Similar to Cono- 

 coryphe, but with a lobe in front of the 

 glabella. Oambrian ; Europe and North 

 America. 



Fio. 1365. 



Fig. 1367. 



Conocoryphe sulzeri Schloth. 

 Without the free cheeks. Cam- Cephalon of>l /ojjs 



-,««. ^ Toö^TN t>rian (Etage C) ; Ginetz, trilineatus Em- 

 Ätops tjmmojis (bigs. 13QA, A; 13b 7). Bohemia. Vi- "^o"«- 



Glabella long, and does not taper. Thorax 

 of seventeen segments. Pygidium small. Lower Oambrian ; North America. 



Family 2. Mesonacidae Walcott {OlenelHdae Moberg). 



Opisthoparia with large cephalon and small, simple pygidium. Facial sutures in 

 a State of Symphysis and usually not to he distinguished. ' Eyes large, the palpebral lobes 

 extending to the glabella. Glabella narrow, sometimes tapering toward the front. 

 Thorax of numerous segments {thirteen to twenty-seven). Lower Cambrian. 



This family is considered by "Walcott to have developed in pre-Cambrian time from sonie 

 annelid-like ancestor by the gradual combination of segments to form the ceplialon and 

 pygidium. A compact, stroiig pygidium, made up of raany segments, does not occur in this 

 family, nor among any of tlie simplest forms of Lower Cambrian Trilobites. The Middle 



