118 THE ROYAL SOCIETY OF CANADA 
rowed ribs which are faintly visible on the pleural lobes of the pygidium. 
This genus is extremely variable, the cephalon and pygidium varying. 
from almost semi-circular to distinctly triangular. The glabella is 
short, the suture intra-marginal, and the hypostoma pointed behind. 
Many of the species are very smooth and very similar in appearance to 
Isotelus. 
In the third line, with the facial sutures marginal, Homalopteon, 
Salter, is intermediate in the matter of segmentation between Ogygopsis 
and Barrandia, Salter, and Barrandia in turn is less smooth than Hemi- 
gyraspis, Raymond, but this can hardly be a correct line of descent, 
as Hemigyraspis occurs in strata much older than those in which the 
other two genera are found. Niobe, Angelin, probably descended from 
the same stem as Hemigyraspis, but is much more primitive in its long 
and distinct glabella, and the possession of ribs on the pleural lobes. 
Niobe is an especially interesting genus, as it seems to represent a 
form which stands near the stem from which all the Asaphide with 
forked hypostomas sprang. By supposing two lines of development 
to have proceeded from Niobe, one in which the primitive characters 
of short head and long glabella are accentuated, and the other in which 
the opposite is developed, we may get all the forms seen in this group. 
Explanation of Plates. 
Plate 1. 
1. Brachyaspis notans, (Billings). A photograph of the type, 
showing the absence of depressed borders, the short and wide cephalon 
and pygidium, and the forward-expanding glabella of an Asaphus. 
The facial suture is, however, marginal in front, and the axial lobe is 
wider than in Asaphus. A phylogerontic feature shown in this species 
is the presence of genal spines. The specimen is from the Richmond 
formation at English Head, Anticosti, and is number 2180 in the Museum 
of the Geological Survey. 
2. Ogygites canadensis, (Chapman). A specimen from the Utica 
at Collingwood, Ontario, to show the strongly ribbed pygidium, the 
narrow axial lobe, and the genal spines. The specimen is exfoliated 
in front, so that the course of the facial suture is not shown. C. G. 8. 
Mus. No. 1934. 
Plate 2. 
1. Onchometopus suse, (Whitfield). Profile view of a specimen 
to show absence of depressed border. This figure also shows the course 
