LEPIDOSIREN— PROTOPTERUS. 17 



some of the extinct cartilaginous fishes, as Cochliodus and Ceratodus" In 1861 Prof. 

 Huxley, 1 while discussing the classification of Devonian Pishes, was led to notice the 

 resemblance between the fins of the Crossopterygian Ganoids and those of Lepidosiren 

 (Protopterus). One passage of considerable historical interest is here cited. 



" Without wishing to lay too much stress upon the fact, I may draw attention to the 

 many and singular relations which obtain between that wonderful and apparently isolated 

 fish, Lepidosiren, sole member of its order, and the cycloid Glyptodipterine, Ctenodo- 

 dipterine, Phaneropleurine, and Ccelacanth Crossopterygidse. Lepidosiren is, in fact, the 

 only existing fish whose pectoral and ventral members have a structure analogous to that of 

 the acutely lobate, paired fins of Holoptj/c/dus, of Dipterus, or of P/ianeropleuron, though 

 the fin-rays and surface-scales are still less developed in the modern than in the ancient 

 fish. The endo-skeleton of Lepidosiren, again, is, as nearly as possible, in the same 

 condition as that of Phaneropleuron, and is more nearly similar to the skeleton of the 

 Ccelacanths than that of any other recent fish ; while, perhaps, it is not stretching the search 

 for analogies too far to discover in the stiff-walled lungs of Lepidosiren, a structure more 

 nearly representing the ossified air-bladder of the Ccelacanths than any with which we 

 are at present acquainted, among recent or fossil fishes. Furthermore, Lepidosiren is 

 the only fish whose teeth are comparable in form and arrangement to those of Dipterus. 

 Though Lepidosiren may not be included among the Crossopterygidae, nor even in the 

 order of Ganoidei, the relations just pointed out are not the less distinct ; and, perhaps, 

 they gain in interest when we reflect, that while Polypterus, the modern representative of 

 the rhombiferous Crossopterygidae, is that fish which has the most completely lung-like of 

 all air-bladders, Lepidosiren, which has just been shown to be, if not the modern repre- 

 sentative of the cycliferous Crossopterygidae, yet their ' next of kin,' is the only fish which 

 is provided with true lungs. These are unquestionable facts. I leave their bearing upon 

 the great problems of zoological theory to be developed by every one for himself.'' 



The later discoveries which seem to have established the position of Lepidosiren and 

 Protopterus among the Ganoids will be related under the heading Ceratodus. Mean- 

 while it will be seen that the Dipnoi are in many of their characters reconcileable with the 

 definition of Ganoidei put forth by Miiller in 1844. The optic nerves do not decussate, 

 the intestine has a spiral valve, the branchiae are free, the ventral fins are abdominal. 

 The most notable differences relate to the heart, for the organs directly related to habitat 

 give scanty information as to affinity, while the other and more striking points of differ- 

 ence which might be cited concern parts which vary greatly among undoubted Ganoids. 

 The Ganoids have rows of transverse valves in the bulbus arteriosus, and but one auricle ; 

 the Protopterini have two longitudinal valves in the bulbus arteriosus, and two auricles. 

 The presence of an auricular septum will not appear of too great importance to those 

 who reflect upon its varying degrees of development in Amphibia. Possessed of an 



1 ' Decades of the Geological Survey,' x, p. 2G. 



