■ PISCES. 13 



hb. Gill opening single, leading to four gill clefts ; jaws coalescent with the 

 skull; a rudimentary opercle ; teeth coalescent forming bony plates. 



( ChirruBras.) Holocefhali, page 24. 



a. Gills free, attached at base only to the gill arches ; gill opening single on 

 each side ; eggs comparatively small and numerous ; no claspers ; mem- 

 brane bones present on head ; cerebral hemispheres not united. ( True 

 Fishes.) . . • Teleostomi, page 25. 



