138 STUDIES IN EVOLUTION 



Family III. Trinucleid.e Barrande. 



Cephalon larger than the thorax or pygidium; genal angles 

 produced into spines. Free-cheeks continuous, almost wholly 

 ventral, carrying the genal spines; suture marginal or sub- 

 marginal. Paired simple eyes, or ocelli, generally absent in 

 adult forms; compound eyes wanting. Segments of thorax five 

 or six in number, with grooved pleura. Pygidium triangular ; 

 margin entire; axis with a number of annulations; limb grooved. 



Ordovician and Silurian. 



Including the genera and subgenera Trlnucleus Lhwyd, Ampyx 

 Dalman, Dionide Barrande, Endymionia ? Billings, Lonchodomus 

 Angelin, Ra2)hiophoriis Angelin, and Salteria ? W. Thompson. 



The leading genera of this family form a tolerably homoge- 

 neous group, although each has sometimes been recognized 

 as characterizing a separate family. Trinucleus and Dionide 

 have a broad pitted border, but this hardly seems of sufficient 

 importance to remove them far from Arnpyx^ since the three 

 genera agree in nearly all important structural details, as 

 the extent and character of the free cheeks, the glabella, the 

 number of free segments, and the character of the pygidium. 

 Lonchodomus and Raphiophorus of Angelin are commonly 

 admitted as sub-genera of Ampyx. 



Both Salteria W. Thompson and Endymionia Billings have 

 been described as sub-genera of Dio?iide Barrande, though 

 there is little positive evidence for this disposition of them. 

 Until more perfect material representing these forms has 

 been described, it will not be possible to decide satisfactorily 

 upon their relationships or place in a classification. There- 

 fore they are left with doubt in the present family. 



Order B. OPISTHOPAKIA, nov. ord. 

 (oiriadev behind, and Tzaptid cheek piece.) 



Free-cheeks generally separate, always bearing the genal 

 angles. Facial sutures extending forwards from the posterior 

 part of the cephalon within the genal angles, and cutting the 



