4 SIRENOID AND CROSSOPTERYGIAN GANOIDS. 



proposed to readmit. The conspicuous differences of their structure from that of the 

 typical Ganoids have been bridged over by the discovery of Ceratodus, as will shortly 

 appear. In other respects Midler's restricted order is coextensive with that of the 

 modern ichthyologist, and the ordinal definition (with the exception of one character 

 noted by Midler himself as provisional) stands unshaken. 



That definition comprehends the following differentiae of Ganoid fishes. 



A Teleostean fish has a two-chambered heart from which ascends the cardiac aorta, 

 dilated a little at its origin, not rhythmically contractile, and separated from the ventricle 

 by a pair of opposite valves. In a Ganoid fish a far more complex structure appears. 

 The dilated origin of the aorta is strengthened by numerous additional muscular fibres, 1 

 which almost entitle it to rank as a third chamber of the heart ; it is, besides, rhythmi- 

 cally contractile. On slitting open this part of the aortic tube (the bulbus arteriosus) 

 rows of valves appear, generally attached to the inner arterial wall transversely, but 

 ranged in longitudinal rows. Polypterus 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. Lepidosteus osseus has from five to eight vertical rows, com- 

 prehending from forty to seventy valves 2 similar to each other and uniformly situated. 

 Amia has five to six vertical series, with two valves in each; Acipenser, four or five 

 vertical series of three or four valves in each ; Polyodon, four vertical rows with three 

 valves in each. The number, arrangement, and relative size of these valves is not 

 quite uniform for the same species. The valves are generally connected together by 

 delicate 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 

 cornubica, the Porbeagle) have valves in the bulbus arteriosus, though less numerous and 

 arranged in three vertical rows. Other Plagiostomi recede still further from the Ganoid 

 type, though the bulbus arteriosus and some trace of the three series of valves are perhaps 

 always to be found. Lepidosiren and Protopterus exhibit the bulbus arteriosus with two 

 longitudinal valves. The two valves found at the ventricular end of the bulb in Teleostei 

 are wanting in Ganoids and Plagiostomi alike. The presence of a contractile bulbus 

 arteriosus furnished internally with valves is thus seen to be a valuable character 

 separating the Ganoids from the Teleostei, Marsipobranchii (Lampreys), and Leptocardii 

 {Amphioxus), though not distinguishing them from Lepidosiren, C/iimcera, the Sharks or 

 Rays. 



A second differential character of Midler's restricted order Ganoidei is to be found 

 in the optic nerves. In the Lampreys and Hags the optic nerve proceeds direct to its orbit 

 from the thalamus opticus of the same side. In a Teleostean fish the nerves decussate, 

 all the fibres from one side of the brain crossing over to the retina of the other side. 



1 In Amia these muscular fibres extend but a short distance up the bulb. 



2 I have found some of the rows in the bulbus arteriosus of Lepidosteus united together and to the 

 wall of the cavity, so as to constitute a kind of longitudinal valve. 



