xnr PHYLUM CHORDATA 211 



2. DISTINCTIVE CHARACTERS AND CLASSIFICATION. 



The Teleostomi are Pisces in which the primary cranium is 

 always complicated by the addition of investing bones, of which 

 a pair of parietals and one of frontals above, and unpaired vomer 

 and parasphenoid below, are the most constant. The chondro- 

 cranium is always more or less ossified by replacing bones, including 

 a supra-occipital, and the upper and lower jaws are both bounded 

 by investing bones. The jaws are connected with the cranium 

 through the intermediation of a hyomandibular, which is probably 

 not homologous with the similarly named element of Elasmobranchs. 

 The dermal fin-rays are formed of bones, and are supported by 

 pterygiophores which may be either cartilaginous or bony, but 

 which always show a great reduction in number as compared with 

 the homologous structures in Elasmobranchs. The primary 

 shoulder-girdle is complicated by the addition of investing bones, of 

 which a large clavicle is the most constant. The pelvic girdle 

 is vestigial or absent. The pelvic fins usually undergo a forward 

 displacement, their position being either abdominal, i.e., between 

 the anus and the pectoral region, or thoracic, i.e., in the pectoral 

 region, or jugular, i.e., under the throat. A dermal exoskeleton 

 is usually present. The intestine may or may not have a spiral 

 valve ; the anus is distinct from, and placed in front of, the urinary 

 and genital apertures. The gills are covered by an operculum 

 supported by investing bones, and the interbranchial septa are 

 reduced or absent, so that the gill-filaments are partially or w r holly 

 free ; the hyoidean gill is reduced or absent. The conus arteriosus 

 is sometimes present, sometimes absent ; when it is absent there 

 is a large bulbus aortse formed as a dilatation of the ventral aorta. 

 The prosencephalon has usually a non-nervous roof ; the optic 

 nerves either form a chiasma or simply decussate. The ova are 

 small ; the gonoducts are either continuous with the gonads, or 

 open anteriorly into the ccelome, or are absent : in the latter 

 case the sexual products pass out by genital pores ; true abdominal 

 pores may be present in addition. Segmentation of the egg is 

 either entire or discoidal ; development is sometimes accompanied 

 by a metamorphosis. 



The Teleostomi are classified as follows : 



ORDER 1. CROSSOPTERYGII. 



Teleostomi in which the pectoral fin consists of a rounded basal 

 lobe supported by endoskeletal structures and fringed by dermal 

 rays. There are no branchiostegal rays. The vertebral column is 

 well ossified, and the caudal fin is diphycercal. The pelvic fins, 

 when present, are abdominal. A spiral valve and a conus arteriosus 

 are present, and the optic nerves form a chiasma. 



