134 STUDIES IN EVOLUTION 



largely upon the species commonly known as H. delphino- 

 cephalus Green, from the Niagara, and H. DeKayi Green, 

 from the Hamilton. The first time the type of the genus, 

 H. Knighti Murchison, is seen he will be puzzled to place it. 

 Similar examples could be multiplied indefinitely, and only 

 show that the type must be taken as the ultimate unit of 

 comparison. 



Diagnoses and Discussions of Orders and Families. 



Order A. HYPOPAKIA, nov. ord. 



(UTTO under, and irapfia cheek piece.) 



Free-cheeks forming a continuous marginal ventral plate of 

 the cephalon, and in some forms also extending over the dorsal 

 side at the genal angles. Suture ventral, marginal, or suhmar- 

 ginal. Compound paired eyes absent; simple eyes may occur 

 on each fixed-cheek, singly or in pairs. 



Including the families Agnostidse, Harpedidse, and Trinucleidse. 



This order includes the groups C and D, or the Ampycini 

 and Agnostini of Salter, and also the family HarpedidaB of 

 that author, which he included in the Asaphini. The special 

 recognition of characters, however, between Salter's groups 

 and the order here proposed is different. 



The presence of a part homologous with the free-cheeks of 

 other trilobites has generally been more or less overlooked 

 in the families of this order. In Trinucleus, Dionide, and 

 Harpes the sutures have been correctly determined by Bar- 

 rande. 3 Likewise, Angelin 2 gave the right structure in 

 Ampyx, but in Agnostus this feature has escaped notice. 

 The examination of extensive series of Agnostus, in the 

 National Museum and in the Museum of Comparative Zool- 

 ogy,* has proved that under favorable conditions of preserva- 

 tion this genus shows a distinct plate, separated from the 

 cranidium by a suture, and it can be compared only with the 



* In the former, through the courtesy of C. D. Walcott and C. Schuchert, and 

 in the latter, of A. Agassiz and R. T. Jackson. 



