[RAYMOND] PARALLELISM AMONG THE ASAPHIDÆ 115 
to a crawling mode of life.* This leads to a form to which Schmidt 
has given the generic name Onchometopus. 
Onchometopus differs from Asaphus in having the glabella less 
prominent, the axial lobe of the thorax wider, the thoracic segments 
flatter, the axial lobe of the pygidium less convex and without rings. 
The anterior portions of the facial suture are also somewhat closer to 
the margin. This leads directly to Brachyaspis, Salter, in which all 
tiaces of outline of the glabella is lost, the pygidium is evenly convex, 
with the axial lobe showing but faintly, and with the anterior portions 
of the facial suture crowded onto the margin. Both these genera show 
their relationship to Asaphus in the lack of the depressed border on 
cephalon and pygidium, the short, wide form of these portions of the 
carapace, and the large prominent eyes. 
Leading on another line of development from Asaphus or an 
Asaphus-like ancestor we have Megalaspides, Brogger, a trilobite very 
like Asaphus, with narrow axial lobe, expanding glabella, no depressed 
border on the pygidium, but with a narrow one on the front of the 
cephalon. One step beyond this is Jsoteloides, Raymond, in which 
both the cephalon and pygidium have depressed borders, the form is 
more elongate, and the glabella is only faintly outlined. Directly 
descended from this is Zsotelus, Dekay, a genus in which the axial lobe 
of the thorax is wide, the glabella and the axial lobe of the pygidium 
so ill defined as to merge into the general surface of the head. Thus, 
on both these lines, the final result has been a smooth trilobite, and 
species of Brachyaspis and Onchometopus are so like Isotelus that the 
three genera can only be separated by careful study with due regard to 
their lines of development. 
This is one of the more striking cases of parallelism, but all these 
genera are within the group in which the hypostomas are forked. Other 
examples are between genera much less closely related, as between 
Isotelus and Asaphellus and Hemigyraspis, or between Nileus and 
Brachyaspis. 
Basilicus, Salter, is the most primitive genus among those which 
have the forked hypostoma, and in many ways it parallels Ogygiocaris, 
Angelin. The pygidium is strongly ribbed, the facial suture is mar- 
ginal in front, the glabella is strongly outlined and shows glabellar 
furrows. From Basilicus, Ogygites seems to have developed. Ogy- 
gites is the name given by Tromelin and Lebesconte to replace the Ogygia 
of Brongniart, that name being preoccupied. The type-species, Ogygia 
guettardi, proves to have a forked hypostoma, and differs from Basilicus 
principally in having the facial sutures meeting in a point in front of 
* See Raymond, Ottawa Naturalist, Vol. XXIV, p. 132, 1910. 
