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DIPNOI. 



Subclass IV. DIPNOI. 



Skeleton partially ossified, with numerous well-developed mem- 

 brane bones. Upper mandibular arch confluent with the chondro- 

 cranium ; gill-clefts feebly separated, opening into a cavity with 

 external cover. Exoskeleton consisting of true bony tissue. In the 

 living forms — optic nerves not decussating, bulbus arteriosus of the 

 heart with series of valves, intestine with a spiral valve, and air- 

 bladder lung-like. 



The dermal or membrane bones of the cranial roof in this subclass 

 exhibit Jittle conformity with the arrangement almost invariably 

 observed in the Teleostomi ; and it seems impossible to apply to 

 them the nomenclature adopted in the case of the latter subclass. 



Dentition of extinct Dipnoi. — 1. Dipterus valenciennesi, Sedgw.&Murch. ; upper 

 and lower jaws, nat. size, xx, upper dental plates ; xxx, lower dental 

 plates ; g, upper dentigerous bones ; n, narial openings. 2. Ctenodus cris- 

 tatus, Ag. ; upper dental plates (somewhat inaccurately drawn, the oral 

 aspect being in reality concave), one-third nat. size. 3. Sagenodus in- 

 &qualis, Owen ; lower dentition, one-half nat. size. 4. Paladaphus insignis, 

 Van Ben. k De Kon. ; lower dentition, one-sixth nat. size. 



