PISCES. 13 



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

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



( Chiirmras.) Holocefhali, page 24. 



aa. 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 



