31 
the exception of one character noted by Muller himself as 
provisional) stands unshaken. 
That definition comprehends the following differentisD of 
Ganoid Fishes. 
A Teleostean fish has a two-chambered heart from which 
ascends the aorta, dilated a little at its origin, but not con- 
tractile, and separated from the ventricle by a pair of opposite 
valves. In a Granoid fish a far more complex structure 
appears. The dilated origin of the aorta is strengthened by 
numerous additional muscular fibres, which almost entitle it 
to rank as a third chamber of the heart ; it is besides 
rhythmically contractile. On slitting open this part of the 
aortic tube (the hulhus arteriosus) rows of valves appear, 
generally attached to the inner arterial wall transversely, but 
ranged in longitudinal rows. Folypterus has three principal 
vertical series, each including nine valves. Alternating with 
these are three incomplete series, each containing five to 
nine valves of less size. In Lejndosteus osseus we usually 
find five vertical rows, comprehending forty valves similar to 
each other, and uniformly situated. Amia has five to six 
vertical series with two valves in each. The number, 
arrangement, and relative size of these valves is not uniform 
for the same species. The valves are generally connected 
together by deKcate tendinous chords, each to the one above 
it in the same vertical row. 
No such structure is known among Teleostean fishes. The 
Sharks (especially Lamna cornubicaf the Porbeagle) display a 
comparable series of valves, though less numerous and 
arranged in three vertical rows. Other Plagiostomi recede 
further from the Ganoid type, though the bulbus arteriosus 
and some trace of the three series of valves is always to be 
found. Lepidosiren and Protopterus exhibit the bulbus arteriosus 
with two longitudinal valves. 
The presence of a contractile bulbus arteriosus furnished 
