72 PALEOZOIC FISHES OF NORTH AMERICA. 
Only the intermandibular bone of this fish has yet been found. In 
size and general aspect this arch with its row of teeth resembles the corre- 
sponding organ in Onychodus Hopkinsi of the Chemung, but may be dis 
tinguished from that at a glance by the very different manner in which the 
teeth are attached to the bony arch. In 0. sigmoides and O. Hopkinsi the 
teeth of the intermandibular crest are expanded at the base with root-like 
projections on either side, which clasp the summit of the arched bone as a 
saddle the back of a horse. As their attachment was only ligamentous, they 
were deciduous, like the teeth of Sharks, and are very frequently met with 
detached from their bony support and scattered over the sea bottom. In 
the present species, however, the mode of attachment is very different, for 
they are implanted in the substance of the arch which sustains them as a 
post is planted in the ground. ‘The mandibular and maxillary teeth of the 
other species of the genus are inserted in the substance of the jaw, and it 
has been a matter of surprise to find the much larger teeth of the inter- 
mandibular crest less firmly attached. From this cireumstance I have sus- 
pected that the teeth of this median row might be erectile, like some of the 
teeth of Lophius. It is evident, however, that in the species now under 
consideration the teeth were much more firmly set, and must break rather 
than bend before a too-powerful opposing force. In the specimen before 
us this accident has happened, since one of the central teeth was broken off 
at about its middle before fossilization. 
The specimen on which this description is based, and the only one 
known, was obtained by Prof. Edward Orton, from the Huron shale, Perry 
Township, Franklin County, Ohio. In age this deposit is intermediate be- 
tween the Corniferous limestone below and the Chemung rocks above, in 
which Onychodus sigmoides and O. Hopkinsi respectively occur; and it is 
remarkable that the present species, intermediate in time between the others, 
should be so radically different in structure. 
Genus ASPIDICHTHYS, Newb. 
A Placoderm fish of large size, having a carapace composed of massive 
bony plates, of which the middle one of the back is similar in form to that 
of Pterichthys, but many times larger, and is covered with large, hemi- 
spherical, smooth, enameled tubercles. 
