Arthrodires 587 



" Arthrothoraci with head-shield hexagonal in outline. 

 Median occipital trapezoidal, margins underlapped conspicu- 

 ously by the external occipitals. Prefrontals meet below pineal 

 plates, thus occluding this element from contact with centrals. 

 The median dorsal plate elongated, terminating in an acute heavy 

 point; no definite ventral keel; its anterior border approaches 

 the head-shield more closely than in related families. Cranio- 

 dorsal joint relatively small. Postero-dorso-lateral large." (?A 

 pair of spines occurs in the pectoral region.) The best-known 

 species is Coccosteiis cuspidatus {decipiens) of the Lower Red 

 Sandstone or Devonian of Scotland. 



The family of Dinichthyide consists of "Arthrothoraci with 

 stout trenchant jaws, whose cutting surfaces have worn away 

 marginal teeth. Plates heavy. Head-shield with conspicuous 

 lateral indentation to form dorsal border of orbit. Preorbitals 

 separated by rostral and pineal elements, the latter passing 

 baclavard between the anterior ends of the centrals. Cranio- 

 dorsal joint conspicuous. Median dorsal shovel-shaped, nearing 

 a stout keel with a large neck and with heavy gouge-shaped 

 terminal. Postero-dorso-lateral relatively small in size." Di- 

 nichthys hertzeri and numerous other species are described from 

 the Devonian and Carboniferous rocks of Ohio. 



The TitanichthyidcB are "Arthrothoraci with slender edentu- 

 lous jaws bearing a longitudinal sulcus. Plates squamous. 

 Head-shield wide, with indentations to form dorsal border of 

 orbit. Cranio-dorsal joint complete, but of relatively small size. 

 Median dorsal with lateral border indented with rudimentary 

 keel and with flat and rounded terminal. Antero-dorso-lateral 

 with an area of overlap on median border." Titanichthys agas- 

 sizi is a gigantic mailed fish from the Lower Carboniferous of 

 Cleveland, Ohio. 



The MylostomidcB are ' ' Arthrothoraci with dental elements in 

 the character of crushing plates. Cranial shield wide, rounded 

 anteriorly, deeply indented in nuchal margin ; orbital rim not 

 apparent in dorsal aspect. Central separated from marginal." 

 Mylostoma terrelli is based on jaws from Cleveland, Ohio. 



The SelenosteidcB are "Arthrothoraci with jaws studded witli 

 cuspidate teeth ; the mandibular rami rounding out anteriorly or 

 presenting diverging tips , bearing teeth in the symphysis . Cranial 



