38 
notable differences relate to tlie heart. The Ganoids have 
rows of transverse valves in the hulhiis arteriosus, and but one 
auricle : the Protopteri have two longitudinal valves in the 
hulhus arteriosus, and two auricles. The presence of an auri- 
cular septum will not appear of too great importance to those 
who reflect upon the variations of both auricular and ventri- 
cular septa presented by nearly allied vertebrata of other 
classes, or eveh by the same species in different stages of 
growth ; moreover this partition is perforated even in Protop- 
terus. The Dipnous type is practically separated from the 
Ganoid type only by the difference in the aortic valves ; for 
the organs directly related to habitat give scanty informa- 
tion as to affinity, and the remaining organs agree, or vary 
only within the limits of variation of undoubted Ganoids. 
The discovery of Lepidosiren and its African ally Protopterus 
has been in every way most important. The homology of 
the swim-bladder of the fish with the lung of the reptile, 
maintained long before by Harvey and Hunter, became clear 
enough now that the important connecting link was supplied 
of a fish's swim-bladder receiving venous and returning 
arterial blood. The definition of the class Pisces became 
more comprehensive, and their affinities to the higher ver- 
tebrata more apparent. From this time zoologists began to 
remark the many points of resemblance between fishes and 
Amphibia (which had hitherto been invariably placed among 
Bep tiles). Hitherto the appearance of lungs in the adult 
had marked in the systems of naturalists the passage from 
the lower to the higher division of cold-blooded vertebrates. 
The absence of gills in all stages was henceforth the dis- 
tinctive character of the higher groups, and zoologists began 
to associate the Fishes with the Amphibia, and the Reptiles 
with the Birds ; a distribution which has since been fortified 
by many arguments drawn from osteology and development. 
The peculiarities of Lepidosiren and Protopterus were so 
