99 



Sub-class II. ^f)Ont(ropter])Q(K (Sharks and Raijs.) 



Skeleton cartilaginous ; skull v,'itliout sutures. Body with 

 medial and paired tins, the hinder part abdominal ; caudal fin 

 with produced upper lobe. Gills attached to the skin by 

 the outer margin, with several intervening gill-openings: 

 rarely one gill-opening only. No gill-cover. No air-bladder. 

 Three series of valves in the bulbus arteriosus. Intestine with 

 a spiral valve. Optic nerves commissurally united, not 

 decussating. Ovaries v.ith few and large ova, which are im- 

 pregnated, and, in some, developed internally. Embryo with 

 deciduous external gills. Males with prehensile ' organs 

 attached to the ventral fins. 



Order 1. HolocepJiala. (Type^ Elephant Fish.) 



One external gill-opening only, covered by a fold of the 

 skin, which encloses a rudimentary cartilaginous gill- 

 cover ; four branchial clefts within the gill-cavity. The 

 maxillary and palatal apparatus coalescent with the skull. 

 (Family 43.) 



Order II. Plag'wstomata. (Type^ Hammer-shark : Tope : 

 Dog-fish: Skate. 



From five to seven gill-openings. Jaws distinct from skull. 

 (Families 44 to 56.) 



Sub-class III. (iJ^clO^tomata, (Type, Lamprey.) 



Skeleton cartilaginous and notochordal, without ribs and 

 without real jaws. Skull not separate from the vertebral 

 column. No limbs. Gills in the form of fixed sacs, without 

 branchial arches, six or seven in number on each side. One 

 nasal aperture only. Heart without bulbus arteriosus. 

 Mouth anterior surrounded by a circular or sub-circular 

 lip : suctorial. Alimentary canal straight, simple, without 

 caecal appendages, pancreas, or spleen. Generative outlet 

 peritoneal. Vertical fins rayed. (Family 57.) 



Sub-class IV. UeptCCattllL (Type, Lancelet.) 



Skeleton membrano-cartilaginous and noto-chordal, ribless. 

 No brain. Pulsating sinuses in place of heart. Blood colour- 

 less. Respiratory cavity confluent with the al)dominal cavity : 

 branchial clefts m great number, the water being expelled by 

 an opening in front of the vent. Jaws none. (One Family 

 only known, No. 58.) 



