32 FAUNA OF THE CORNBRASH. 



Order AOTINOPTERYGII. 



Family Pycnopontidj:. 



Genus MACROMESODON, nov. 



The genus Mesodon was founded in 1851 % Wagner on teeth which the author 

 denned as " long oval, with the surface superficially hollowed in a longitudinal 

 direction and with the slope of the hollow finely furrowed." The name Mesodon 

 referred to the teeth occupying a middle position in this respect between Gyrodus 

 and Pycnodus. It was found, however, when complete fishes were obtained, that 

 the Jurassic Pycnodonts were in many respects quite distinct from the Tertiary 

 Pycnodont, and the name Pycnodus was therefore confined to the latter, and 

 Mesodon to the former. The above quoted definition is not, however, the prin- 

 cipal diagnostic of these Mesozoic forms, and it is therefore fortunate that a new 

 name is required according to priority, since the name already suggested was 

 preoccupied by Rafinesque in 1831 for a member of the Helicidge, and is actually 

 in use. That suggested refers to the large size of the central vomerine teeth, 

 and the genus may be thus defined : 



Teeth smooth and with feeble indications of rugae; vomerine teeth arranged 

 in five longitudinal series, the lateral pairs being often irregular; splenial den- 

 tition comprising one principal series of teeth, with three or more outer series 

 and one or two inner series, usually irregularly arranged. 



Macromesodon bathonicus (Sauvage). Plate I, fig. 10. 



1880. Mesodon bathonicus, Sauvage, Bull. Soc. Greol. France [3], vol. viii, p. 527, pi. xix, fig. 1. 

 1867. Pycnodus affinis, Sauvage, Cat. Poissous Form. Second. Boulonnais, p. 15, pi. ii, fig. 6. 



Shiatype. — This is a vomerine bone with the teeth in position, so far as pre- 

 served. " Four teeth in the principal range, of large size, transversely elongated, 

 a little larger on the outside than on the inside, hinder teeth a little more than 

 twice as broad as long, the anterior teeth a little narrower. The internal range 

 is furnished with five teeth, practically oval, small, 2-3 mm. diam. These teeth 

 show in their centre a mammillated surface surrounded with irregular folds 



