112 THE ROYAL SOCIETY OF CANADA 
These primitive characters appear in the genera in various com- 
binations. Ogygiocaris and Ogygopsis have 1, 3, 4, 5, 6, 7,8; Asaphus 
and Symphysurus have 2,3; 5,6,7; Ogyginus has 1, 3,6, 7,8; Basilicus 
has 1, 5, 6,7, 8; Niobe has 1, 3, 5, 6,7, and so on. On the other hand, 
certain species of Zsotelus lack all these characters, Nileus has only 2, 3, 
5; Ptychopyge only 7 and 8; Hemigyraspis has 5, 6, 7, 8; while Asa- 
phellus has only 7 and 8. Judged merely by the above standards, 
Ogygopsis, Ogygiocaris, Ogyginus, Asaphus, Niobe, Symphysurus, and 
Basilicus are more primitive than Jsotelus, Nileus, Ptychopyge, Hemi- 
gyraspis, and Asaphellus. 
Hypostoma. 
The hypostomas of various species of asaphids have been studied 
in detail by Brôgger, who finds three distinct types. One, typified by 
Megalaspis and Ogygiocaris, is pointed behind; the second, found in 
Niobe insignis, is quadrate; and the third, in Asaphus, Isotelus, and 
other genera, is extended behind into two long, large prongs. The 
hypostomas of trilobites do not change rapidly, and are therefore im- 
portant in marking the larger subdivisions in classification. The 
Asaphidæ can thus be divided into two great groups, according as the 
posterior margin of the hypostoma is entire or forked. The genera in 
which the hypostoma is entire are; Ogygopsis, Ogygiocaris, Ogyginus, 
Asaphelina, Homalopteon, Barrandia, Homoglossa, Ptychocheilus, Niobe 
(partim), Hemigyraspis, Asaphellus, Megalaspis, Platypeltis, Symphy- 
surus, Illænurus, Nileus, and Vogdesia. In the following the hypostoma 
is forked; Basilicus, Ogygites, Ptychopyge, Pseudasaphus, Megalaspides, 
Isoteloides, Isotelus, Asaphus, Brachyaspis, Onchometopus, and Niobe 
(partim). 
It will be noted that in Niobe we have all transitions between the 
two types. In Niobe insignis the hypostoma is entire, the posterior 
margin being straight. In Niobe emarginula there is a slight indenta- 
tion, in N. leviceps it is deeper, and in N. frontalis and in N. explanata 
the forks are well developed. In geologic position, Niobe insignis is 
the most ancient and N. frontalis the most recent of these species,and 
the others appear in the order of the development of the forks of their 
hypostomas. Brôgger has shown this in a table on page 52 of his re- 
markable paper on the hypostomas of the Scandinavian Asaphidæ. 
N. frontalis. Zone of Megalaspis limbata. À 
N. explanata. Lower part of same zone. f 
N. leviceps. Phyllograptus zone. Beekmantown. 
N. emarginula. Ceratopyge limestone. 1 
N. insignis. Ceratopyge shale. J 
Lowville& Black River. 
Top of Cambrian. 
