19 



jeering freely into the arterial cavity. If these internal valves were 

 straight, they would resemble the single thicker valvular process which 

 occupies the elongated bulbus arteriosus of the Siren ; here, however, they 

 follow the spiral turn of the aorta." 



This structure of the arterial trunk has perhaps more analogy to that of 

 the Amphibians than to that of fishes ; but the two longitudinal valvular pro- 

 jections or ridges of the arterial trunk of Protopterus appear, nevertheless, 

 not to be fundamentally different from the longitudinal rows of valves 

 found in the trunk of the Ganoids, especially of the Amioids. By the 

 coalescence of the valves of each of the respective rows in the bulbus 

 arteriosus of the Amice, analogous ridges would be formed. 



Another distinction from all other fishes is produced by the abnormal 

 suspension of the scapular arch only to the exoccipital bones of the 

 Lepidosirenoids. 



The importance of both of the above mentioned peculiarities is undoubt- 

 edly of considerable value, but it is very doubtful whether they alone are 

 of sufficient importance to authorize the separation of the Lepidosirenoids 

 from the subclass of Ganoids. 



The Lepidosirenoids have many affinities with the Ganoids. The brain 

 bears a considerable resemblance to that of the Polypteroids and the 

 Acipenseroids ; the persistent notochord, combined with ossified vertebral 

 elements, recalls to mind the similar structure which prevailed in many of 

 the older representatives of the subclass ; the strict homology of the pul- 

 monary sacs and communication with the intestinal canal of Lepidosiren 

 and Polypterus ; the presence of a spiral valve in the intestines ; the ab- 

 dominal position of the ventral members, are all characters which indicate 

 the close alliance of the Dipnoi and Ganoids. Cycloid and regularly 

 imbricated scales are found on the recent Amioids of America, and in 

 extinct representatives of the order. We appear then justified in consider- 

 ing the internal structure of the arterial trunk, and the mode of attachment 

 of the scapula to the cranium, as of secondary value. As the Dipnoi agree 

 in all other essential respects with the Ganoids, we will then at least 

 provisionally consider them as belonging to the same great subclass for 

 which the latter name may be retained.* 



Professor Owen has for the most part adopted the classification proposed 

 by Miiller, but has not recognized the subclasses, of which six were named 



*It was not until some time after the completion of the above remarks that 

 the observation of Miiller respecting the reference by Agassiz of the genus 

 Lepidosiren to his order of Ganoids was noticed. The reasons assigned by 

 Agassiz for that act have not been stated. We do not know where that natural- 

 ist has published his observations. In his recent Essay on Classification, the 

 Ganoids are simply accepted as a class with three undoubted orders called 

 Coelacanths, Acipenseroids and Sauroids; and three doubtful ones, the Silu- 

 roids, Plectognaths and Lophobranches. It can scarcely be intended to refer 

 the Lepidosirenoids to any of those orders. We remain, therefore, in doubt as 

 to the position that Professor Agassiz would assign to them. 



