INTRODUCTION. xxiii 



Sub-class III. — GANOIDEI has the skeleton more or less ossified ; the scales often are 

 hard and polished (701/0?, "brightness"); ventral fins, when present, are abdominal; the 

 intestine has a spiral valve. It is divided into two orders : 



I. — HoLOSTEi. Body covered with scales ; skeleton bony. 



A. Scales cycloid, i. Ajuiidcc (Bowfin or Mudfish of the fresh waters of North America). 



B. Scales ganoid. 



(a) Fins without fulcra. 2. Polyptcrida. Fresh waters of Central and Western Africa. 

 {b) Fins with fulcra. 3. Lepidosteida^ as the Bony Pike (L. oiseus) of North America. 

 II.— Chondrostei. Skin naked, or with osseous bucklers; skeleton partly cartilaginous. 



A. Mouth small, transverse, inferior. Acipenserida (Sturgeons). 



B. Mouth lateral, very wide. Polyodontidce (Spoonbill Sturgeon of the Mississippi and 

 tributaries). 



Fresh-water representative of A is met with in the Sturgeon. 



Sub-class IV. — CHONDROPTERYGII. In this sub-class the fish have a cartilaginous 

 skeleton, and the skull is without sutures ; the tail has a produced upper lobe ; the gills are 

 attached to the skin by the outer margin, with several intervening gill-openings ; rarely with 

 one gill-opening only ; no gill-cover ; no air-bladder ; intestine with a spiral valve ; ovaries 

 with few and large ova impregnated internally, and in some cases developed internally. Males 

 with prehensile organs attached to the ventral fins. Contains two orders. 



I. — HoLOCEPHALA. One external gill-opening only, as in the Cliiincera vioiistrosa (Arctic 

 Chimcera). 



II. — -Plagiostomata. From five to seven gill -openings. 



Sub-order 1. — Selachoidei. Gill-openings lateral (Sharks and Dog-fish). 



Sub-order 2. — Baioidci. Gill-openings ventral (Rays). 



Of this sub-class there is no British fresh-water representative. 



Sub-class V. — CYCLOSTOMATA, contains fishes whose skeletons are cartilaginous, and 

 notochordal, which have no ribs, no real jaws ; whose skull is not separate from the vertebral 

 column, and which are limbless ; the -gills have no branchial arches, but are in the form of 

 pouches or sacs, generally seven in number on each side of the neck ; there is only one nasal 

 aperture ; and the heart has no bulbus arteriosus. The mouth is suctorial in the mature 

 form, or crescent-shaped in the larval form. The alimentary canal is simple, straight, without 

 caecal appendages, pancreas, or spleen. 



There are two families, viz : 



I.- — Pctromyzontida, in which the nasal duct terminates blind, not penetrating the palate; 

 as in the Lampreys. 



2. — MyxinidcE, in which the nasal duct penetrates the palate, (Glutinous Hag). 



In the sub-class VI. — LEPTOCARDII, the only representative, I believe, is the Lancelet 

 {Branchios/oiiia laiiccolattuii,) occasionally found on the English southern coasts. This fish 

 occupies the lowest scale amongst vertebrate animals ; its skeleton is membrano-cartilaginous 

 and notochordal ; it has no ribs, no brain, pulsating sinuses in place of a heart ; the blood is 

 colourless ; the respiratory cavity is confluent with the abdominal cavity, the branchial clefts 

 are numerous, and the water is expelled by an opening in front of the vent. There are no 

 jaws. 



The British ichthyological fauna numbers about two hundred and fifty species, of which 

 about fifty-five are either permanently or periodically inhabitants of fresh water. With the 

 exception of the Sturgeon and the Lampreys, all the fresh-water fishes of the British Isles 

 belong to the great sub-class Teleostei, and to the orders, I. Acanthopterygii, which contains 

 six species ; III. Anacanthini, two species ; and IV. Physos'.oini, all the remaining species, 

 represented by the families Cyprinida, Clupcida, Esocidce, Salinoiiidcc, and Jldiiraiiidcr. 



