HETERODONTI 147 



small inner and larger outer plate ; Paecilodus, M'Coy; Deltoptychius, Ag., 

 both plates join to a single large plate on each side ; Diplacodus, Davis 

 all from the Carboniferous of Europe. Xystrodus, Ag., Sandalodus, N. 

 and W. ; Carboniferous of N. America and Great Britain. 



Family EDESTIDAE. Certain peculiar coils of teeth in a single row 

 have been found, which have been variously interpreted by palaeontolo- 

 gists as compound spines, as the armature of a twisted snout (Karpinsky 

 [256]), or as a spirally coiled row of median symphysial teeth of the 

 lower jaw of a fish allied to the Cestraciontidae (A. S. "Woodward [503], 

 Eastman [127]). 



The last interpretation certainly is most in harmony with what we 



FIG. 107. 

 Upper jaw of Heterodontus (Ccstmclon) Philifpi, Lac., the Port Jackson shark. (After Owen.) 



know of the succession and position of the teeth in other Selachii. 

 Already in the Cochliodonts the inrolling of the worn edge of the com- 

 pound tooth-plates is seen ; and the little Devonian Selachian Protodus 

 seems to show the initial stage in the formation of a spiral coil of a single 

 row of teeth (A. S. Woodward). In Campodus the median teeth, which 

 .are not much compressed, form a short coil of about thirteen teeth, 

 and there are series of lateral teeth resembling those of the Cestraciont 

 Orodus (Eastman). Only the median coil is known in the more 

 modified Edestus and Helicoprion. Here the teeth are much compressed, 

 and there may be as many as 150 in a single coil (Fig. 108). 



Protodus, A. S. W. ; Devonian, England and Canada. Campodus, 

 Kon., Carboniferous ; Edestus, Leidy, Carboniferous ; Helicoprion, Karp. ; 

 >Carboniferous and Permian Europe and North America. 



