[RAYMOND] PARALLELISM AMONG THE ASAPHIDÆ 113 
Within the two great groups indicated by the difference in the form 
of the hypostoma, the course of development seems to have been from a 
strongly segmented to a smooth form, and this has given rise to species 
which are very similar in general aspect, though not closely related. 
As evidence of this similarity it may be noted that species of Megalaspis, 
Asaphellus, Onchometopus, and Brachyaspis are commonly referred to 
Isotelus, and almost any asaphid is referred to Asaphus. 
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Geologic Range. 
The asaphids appear earlier in America, and have a longer range, 
than in any other country. In the Middle Cambrian at Mount Stephen, 
Ogygopsis is abundant. At the top of the Upper Cambrian, Jilenurus, 
Symphysurus, Asaphellus, Hemigyraspis, and Megalaspis occur. In the 
Beekmantown, the asaphids are less varied, as only Jsoteloides and 
Nileus are present. In the Chazy, Isotelus, Isoteloides, Bastlicus, On- 
chometopus, Vogdesia and Nileus occur. In the Lowville and Black 
River we find Onchometopus, Isotelus, Isoteloides, Basilicus, and V ogdesia. 
In the Trenton, there are, so far as is now known, only /sotelus and 
Isoteloides. In the Utica, Zsotelus and Ogygites. In the Richmond, 
Isotelus, Onchometopus, Brachyaspis, and probably Ogygites. 
It will be noted that all the Cambrian asaphids belong to genera 
having entire hypostomas, and that the first genus with a forked 
hypostoma appears in the Beekmantown. In faunas younger than 
the Beekmantown, asaphids with entire hypostomas are rare, though 
they do continue as late as the Black River. Asaphids with forked 
hypostomas begin to be common in the Chazy, and continue throughout 
the remainder of the Ordovician. 
In the British Isles, the oldest asaphids are found at the top oi the 
Cambrian, (Tremadoc), where Symphysurus, Platypeltis, Ogygiocaris, 
Asaphellus, Hemigyraspis, and Niobe occur. In the Arenig, Niobe 
and Barrandia are present. In the Landeilo, Ogygiocaris, Ogyginus, 
Barrandia, Homalopteon, Basilicus, and Isotelus represent the family. 
In the Caradoc, I[sotelus, Brachyaspis, and Basilicus occur. 
In the Christiania District of Norway, Symphysurus, Nileus, Niobe, 
and Megalaspis occur in 38a, which corresponds to the Tremadoc or the 
top of the Cambrian. In 3b, (Arenig, Beekmantown), Symphysurus, 
Megalaspis, Niobe, and Megalaspides are. present. In 3c, (Lowville 
and Black River), the asaphids are Symphysurus, Nileus, Niobe, Megal- 
aspis, Ptychopyge, and Asaphus. In 4, Ogygiocaris occurs. 
In Russia, no asaphids are known from deposits older than the 
Ordovician. In the lower zones, B1 to D3, (Black River group), Mega- 
laspis, Megalaspides, Asaphus, Pseudasaphus, Ptychopyge, Nileus, 
Sec. IV., 1911. 8. 
