LEPIDOSTEI. 179 



teriorly. The air-bladder is single and opens into the pharynx 

 dorsally. It has a continuous central cavity opening on each 

 side into lateral chambers which are placed in the thickness 

 of the wall. 



In Lepidosteus the cerebrum (Fig. 106) is divided into two 

 parts, an anterior and a posterior. The anterior part tapers 

 in front into the olfactory lobes and is double ; the posterior 

 part is single and its ventricle possesses a thin roof like that 

 of the thalamencephalon, with wilich it is continuous. The 

 dorsal part of its side walls, where they pass into the roof are 

 everted and thickened and form the prominent posterior cerebral 

 lobes (Fig. 106). The anterior part of the roof of the thalamen- 

 cephalon is produced dorsalwards into a large thin-walled 

 vesicle which projects just in front of the pineal body. The 

 cerebellum is of medium size and has a forwardly projecting 

 lobe. 



FIG. 107. Dapediits politus, restored with scales removed, quarter natural size, Lower 'Lias 



(from British Museum Catalogue). 



Fam. 1. Lepidosteidae. Body covered with thick, rhombic, ganoid 

 scales, vertebral column completely ossified, vertebrae opisthocoelous ; 

 tail heterocercal ; the snout is much elongated. Lepidosteus Lac., gar- 

 pikes. Fresh-waters of N. America and Cuba, one species is known 

 from China. Sluggish habit, voracious, valueless as food. 



Extinct families. 



Fam. 1. Stylodontidae. All fins with fulcra. Jaws and vomer with 

 several rows of teeth. Vertebral column composed of half vertebrae or 

 of ring-vertebrae. Upper Permian to Cretaceous. Acentrophorus Tra- 

 quair. Semionotus Ag., Dapedius de la Beche, a deep-bodied fish 



(Fig. 107). 



