410 STUDIES FOR STUDENTS 



rhombic scales and in the Teleosts of horny, cycloidal scales. 

 It is readily shown that these characters are of the most super- 

 ficial nature and the condition of one group is easily found in 

 the other, still as a general thing it may be said that a majority 

 of the older forms had the rhomboid type of scale and the 

 modern forms have the horny type. Bashford Dean discusses 

 the relationships and descent of the Teleostomes in the follow- 

 ing words, p. 145 : "Johannes Miiller, when separating Ganoids 

 from Teleosts, recognized clearly even at that early date (1844) 

 that the majority of the structural differences of these forms 

 were bridged over in exceptional instances ; there were thus 

 Teleosts with bony body plates, as well as, it was afterwards 

 found, a Ganoid, (Amia) with herringlike cycloidal scales. 

 But he believed that three structural characters of the Ganoids 

 separated them constantly from all Teleosts, and warranted the 

 integrity of the groups." 



These distinguishing characters were: — 

 I. A contractile arterial cone, containing rows of valves. 



II. An intestinal spiral valve. 

 III. The interfusion (chiasma) of the optic nerves. 



It was not until these differences were shown to be of little 

 morphological importance that the two groups were merged in 

 that of the Teleostomi (Owen, 1866). Thus transitional char- 

 acters of the arterial cone of Butrinus were discovered by Boas. 

 The Teleost, Cheiroc entries was found to present Ganoidean intes- 

 tinal characters, and the optic chiasma, as Wiedersheim demon- 

 strated, could no longer be regarded as of taxonomic or 

 morphological value. 



The descent of the Teleostomes, like that of the other 

 groups, has long been a matter of speculation. Their affinities 

 with the Dipnoans ?re generally admitted ( Gunther, Gegenbaur, 

 Haeckel, Smith-Woodward ). Rabl derives them directly from 

 a Selachian stem, regarding the Dipnoans as later evolved 

 Ganoidean forms. Beard, on the other hand, even goes so far 

 as to entirely separate the Teleostome stem from that of the 

 shark, lungfish, an amphibian, deriving it with a close kinship 



