ORDER GANOIDEI. 



985 



a large size. The vomerine teeth (fig. 924) of the middle row are 

 elliptical, and much larger than either of the others, while there 

 are usually three rows on either side of the mandible, although 

 these are occasionally increased to four. The dorsal fin extends 

 in advance of the anal. This genus ranges from the Gault to the 

 Chalk, and is widely spread through Europe. In Mesodon we 

 have a genus ranging from the English Lias to the Lower Kimer- 

 idgian of Bavaria, and not improbably also represented in the 

 Lower Greensand. Here the vomerine teeth of the middle row 

 are larger than the teeth of the other rows, which are exceedingly 

 small, and in the outermost line have a wart-like surface. Allied 

 genera are Mesturus and Athrodon from the Kimeridgian of the 

 Continent. 



Stemmatodus (fig. 925), which comprises two small species from 

 the Lower Cretaceous of Italy, is characterised by the great length 



Fig. 925. — Stemmatodus rhombus ; from the Lower Cretaceous of Italy. 



of the dorsal and anal fins, and also by the concave surfaces of the 

 molariform teeth, which are of subequal size. Microdon is another 

 allied genus, ranging in Europe from the Kimeridge to the Purbeck. 

 The arrangement of the teeth, and the peculiar structure of the 

 heads of the ribs are shown in fig. 923. In Gyrodus we seem to 

 have the most specialised members of the entire family, all of them 

 being characterised by the sculptured crowns of the molariform 

 teeth. The vomer is very narrow, and has the teeth subcircular, 

 those of the middle row being much the largest ; while in the man- 

 dible there are four rows on either side, of which the first and third 

 are the larger. This genus is especially abundant in the Lower 

 Kimeridgian lithographic limestone of Bavaria, but it also ranges 

 upwards into the Chalk of Sussex, and downwards into the Lower 

 Jurassic Stonesfield Slate. Finally, Coccodus is founded on an im- 

 perfect specimen from the Chalk of the Lebanon really belonging 



