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Order CHONDROSTEI, Muller. 



The body is sometimes naked, but in most of its species is covered with 

 more or less interrupted rows of osseous or ganoid plates of irregular 

 form. There are also many smaller plates or tubercles scattered on differ- 

 ent parts of the body. There are no true branchiostegal ray*.* 



The vertebrae and their elements are cartilaginous. The skull is also 

 generally cartilaginous, but is sometimes imperfectly ossified anteriorly. 

 The scapular arch is suspended by two processes to the paroccipitul and 

 to the mastoid bones ; it supports two well developed pectoral fins. The 

 ventral fins are also furnished with several rays. The bulb of the aorta 

 is furnished with several longitudinal rows of valves. 



This order embraces two families of recent fishes ; the Sturionoids of 

 the northern portions of the old and new worlds, and the Polyodontoids 

 peculiar to the rivers of the central parts of North America. 



Order DIPNOI Muller. 



The body is elongated, and covered with regularly imbricated cycloid 

 scales. The centra of the true vertebrae are cartilaginous, the notochord 

 being persistent. The neurapophyses and haeinapophyses with their re. 

 pective spines are osseous. The continuous vertical fin or fold, encircling 

 the posterior part of the body, is sustained by articulated rays, immediately 

 connected with the spinous processes of the neurapophyses and haemapo- 

 physes. The scapular arch is suspended only to the exoccipital bone, 

 and supports on each side a simple unjointed or articulated ray. The 

 ventral fins are also represented by a simple ray on each side. The bulb 

 of the aorta is furnished internally with two spiral ridges or valves. 



Of this order, only a single family, embracing two genera, is known. 

 The genus Lepidosiren, of Natterer, is confined to South America, and the 

 Proiopteri, of Owen, to Africa. 



Subclass ELASMOBRANCHII Bon. 

 The Elasmobranchii of Bonaparte and Muller have the endo-skeleton 

 or vertebral column and skull cartilaginous, or very imperfectly ossified. 

 The exo-skeleton is developed in the form of placoid granules. The 

 brain is much more complex and highly developed than in the true fishes: 

 the optic nerves are connected by a commissure, but do not cross each, 

 other. The bulbus arteriosus or aorta has a thick muscular coat, and is 

 provided with at least two rows of semilunar valves. The intestine has a 

 spiral valve. The males are provided with the so-called " claspers," which 

 are present as appendages to the posterior edges of the ventral fins ; fe- 

 cundation is effected by copulation. The branchial apertures are usually 

 five in number, and are generally all external. There is no air bladder. 



*This character must be accepted with some modification ; the ceratohyal of 

 the representatives of the genus Polyodon has on each side an appendage 

 which is evidently homologous to the branchiostegal rays of other fishes. 



