930 



CLASS PISCES. 



which is found both in Europe and North America, the teeth are of 

 the same general type, but have more compressed crowns, with long 

 undivided roots. Again, in Petalodus, which has the same distribution 

 as the preceding, the teeth (fig. 852, 4) are much elongated trans- 

 versely, and compressed from before backwards — the crown being 

 petal-shaped, with a smooth or slightly crenulated cutting-edge. The 

 nature of the arrangement of the entire series is, however, unknown. 

 The teeth of Ctenoptychius (fig. 852, 6) are distinguished by the 

 coarser denticulation of their cutting-edge — those of Callopristodus 



MtMtl^^ ^v»»tt\»w. 



Fig. 852. — Spines and teeth of Carboniferous Elasmobranchei. i, Nuchal spine of Pleuracan- 

 thus IcEvissimus; 2, Fin-spine of Gyracanthus ; 3, Do. of Sphenacanthus', 4, Tooth of Petalodus 

 acmninatus, seen from the side ; 5, Do. of Psammodus, seen from above ; 6, Do. of Ctenopty- 

 chius. 1-3 are reduced. 



differing from the latter by the absence of enamel-folds at the base 

 of the crown, and also by the nature of the root. It is probable 

 that both these genera occur in North America as well as in Europe. 

 In Polyrhizodus {JDactylodus), of both Europe and North America, 

 the teeth are extremely stout, with low crowns, usually having a 

 sharp cutting-edge devoid of crenulations, and a large root divided 

 into a number of rootlets. Other allied genera from Europe are 

 known as Glossodus and Mesolophodus. 



Family Pristodontid^:. — This family is only known by the 

 genus Pristodus, of the European Carboniferous, in which the crown 

 of each tooth is thin, plate-like, and symmetrical, with hollows corre- 

 sponding to elevations in the opposing tooth of the opposite jaw. It 

 has been suggested that each jaw carried only a single tooth. 



Family Squatinid^. — With this family we come to a group rep- 

 resented at the present day by the Monkfish, of which the skele- 



