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Order HOLOGEPHALI Bon. 



In the species of this order, there is only one branchial fissure on each 

 side as in true fishes. 



No representatives of the Holocepliali have yet been detected on the 

 Eastern American Coast. 



Subclass DERMOPTERI Owen. 



The fourth subclass may provisionally embrace both the Cyclostomi or 

 Marsipobranchiiaud the Pharyngobranchii or Cirrhostomi. Thus enlarged, 

 it corresponds to the Dermopteri of Owen, and may retain that name. 

 The body is much elongated and either subcylindrical or compressed. 

 The endoskeleton is very rudimentary and cartilaginous, and in the order 

 of Pharyngobranchii, there is no distinct head. The pectoral and ventral 

 fins are both absent. The skin is entirely naked and mucous, and the fins 

 are only folds of the skin. There are no pancreas nor air-bladder. The 

 olfactory organ and nostril are single. 



There are two orders. 



Order HYPEROARTII, (Bon.) Mull. 



The body is invariably greatly elongated and subcylindrical, or anguil- 

 liform. The head is distinct. The myelon, or medulla spinalis, is descri- 

 bed by Owen as being depressed and flattened, " of opaline subtranspa- 

 rency, ductile and elastic." The bulbus arteriosus is absent, but there 

 are two opposite valves at the origin of the branchial vessel, as in the 

 Teleostei, The branchiae are purse-shaped and inoperculate ; there are 

 seven in number on each side. Each receives the streams of water for 

 the aeration of the blood through short tubes, entering from a median 

 canal which is below and distinct from the oesophagus, and which termi- 

 nates behind in a closed wall, and, according to Professor Owen, commu- 

 nicates with the fauces anteriorly "by an opening guarded by a double 

 membranous valve." 



This order answers to the order of the same name of Muller, and the 

 family of Petromyzontoids of the order Dermopteri of Owen. It em- 

 braces, on our coast, the "lampreys," or " lamper-eels " (Petromyzon). 



Order HYPEROTRETI, (Bon.) Muller. 



The representatives of this order resemble, in most respects, those of the 

 Hyperoartii, chiefly differing in the respiratory apparatus. The branchia; 

 are bursiform and fixed, receiving the streams of water directly from the 

 oesophagus through short tubes communicating with each sac. The water 

 is discharged through tubes which either severally open externally, or 

 into two lateral and longitudinal canals, directed backwards and dis- 

 charging by as many orifices on each side of the median line of the ven- 

 tral surface. 



