STRUCTURES OF ARTHRODIRAN * ‘933 
extremity of the notochord.” Gill arches and opercula are 
not definitely known. The mandible was attached directly 
to the skull (autostylic). The jaws were shear-like, their 
margins usually with pointed teeth, whose bases fuse with 
the tissue of the jaw and constitute dental plates. In 
all forms, as in Dinichthys (Frontispiece), there appear to 
have been three pairs of these “ plates,” those forming the 
rim of the mandible below, and those of the vomerine 
and palatine regions (“premaxillary” and “maxillary’’) 
above.* This arrangement of the dental plates somewhat 
resembles the Dipnoan’s. Those of the Arthrodiran, how- 
ever, appear to have been movable, and suggest a dental 
condition elsewhere unknown among vertebrates. 
Fig. 133.— Restoration of Dinichthys intermedius, Newb. X %. Cleveland 
Shales, Ohio. 
The body armouring of dermal plates is characteristic of 
the group. A carapace, cape-like in shape, begins at the 
head angle and broadens out backward and dorsally 
towards the median line. It consists of a single median 
spade-shaped element, which forms the strong ridge of the 
back, and a flanking of lateral plates, all compactly joined. 
The rigid shield that is thus formed is movably connected 
with the head; an elaborate joint, formed on either side 
between the anterolateral dorsal plate, Fig. 131, ADZ, and 
the “ epiotic,”” HO,— whence the name Arthrodira, — must 
* According to Dr. Clark, an additional symphysial pair of dental plates 
was present in both upper and lower jaw (Dinichthys). 
