STRUCTURES OF ARTHRODIRAN 



133 



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 5$. 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, ADL, and 

 the **epiotic," EO, — 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). 



