XX] CLASSIFICATION 513 



Order II. Holocephali. Chondrichthyes in which there is an 

 opercular flap springing from the hyoid arch and covering the gill- 

 slits, in which the skin is naked, the placoid scales being restricted 

 to the frontal tentacle of the male and to spines in front of the 

 unpaired fins. The upper jaw is fused with the skull and the noto- 

 chordal sheath is not divided into centra. 



Ex. Ckimaera, JRhinocMmaera, Harriotta, Cattorhynchus. 



Sub-class II. OSTEICHTHYES. Pisces in which true bone is 

 present, at least in the form of dermal plates covering the head, and 

 of flat scales covering the body and connecting together the bases 

 of the placoid denticles if these are present. Bone is also present 

 in every case as a partial or complete investment of the cartilage 

 of the skull. An air-bladder is developed as an outgrowth of the 

 alimentary canal behind the gill region. The septa between the 

 gill-sacs are narrowed so that these become mere slits and a gill- 

 cover or operculuin is developed from the hyoid. The nostril is 

 divided into two openings, the eggs are small and the young 

 emerge from them as larvae which undergo a long period of 

 development before becoming adult. 



Order I. Teleostei. Osteichthyes in which the notochord is 

 surrounded and obliterated by bony amphicoelous centra developed 

 from the connective bone outside the proper notochordal sheath. 

 Between the centra the notochord persists. . No intercalary arches 

 are developed. The cartilage of the skull is covered by and largely 

 replaced by bone and in particular a large median supra-occipital 

 bone is always developed. The upper jaw is amphistylic articulating 

 with the skull in front and supported by the hyoid behind. No 

 splenial bone is developed in the lower jaw and there are no clavicles 

 in the pectoral girdle. The tail is homocercal. The optic chiasma 

 in the brain is resolved into two distinct nerves. The conus is 

 amalgamated with the ventricle, and only one transverse row of pocket 

 valves persists. There is no special valve in the intestine. The 

 testis has a special duct opening by a special pore and in both sexes 

 excretory ducts, genital ducts and alimentary canal open to the 

 exterior by separate apertures. 



Sub-order 1. Clupeiformes. Teleostei in which the pelvic 

 fins are abdominal in position, the air-bladder has an open duct, the 

 coracoid develops a mesocoracoid branch and in which there are no 

 hard spines in the fin rays. The maxilla bears teeth and forms 

 part of the gape, 



S. & M. 33 



