Phylogeny of the Tehostomi. 333 



originally distinct cartilaginous rays is clearly indicative of a 

 more primitive condition of these structures than can be found 

 in any other living vertebrate animal." 



The Chondrostean ventral fin having been thus described 

 as principally composed of a series of basal cartilages 

 (baseosts) supporting the dermal rays, articulated internally 

 to another series of cartilages (axonosts) which exhibited 

 some fusion anteriorly, it was inexeusably careless of Cope * 

 to propose a classification ignoring this, his order Podopterygia 

 (/. e. Chondrostei) being characterized as possessing median 

 fins with numerous axonosts, pectoral without axonost and 

 rudimentary baseosts, and ventral with one axonost and 

 several baseosts. In Smith Woodward's classification f, 

 which is based on that of Cope, the structure of the paired 

 fins in the Chondrostei has also remained unappreciated. 

 Finally Traquair J, in discussing the evolution of fishes, 

 whilst paying considerable attention to the paired fins of 

 Crossopterygii and Dipneusti, does not even think them 

 worthy of notice in the comprehensive order Actinopterygii. 

 So that it would almost seem as if the structure of the paired 

 fins in the Chondrostei, of the highest importance in any 

 discussion as to the affinities of that order and of the very 

 greatest interest as evidence in favour of the lateral fin fold 

 theory, although well known to the morphologists, is in 

 clanger of being forgotten by the systernatists. 



The ventral fins of Psepkurus gladius are even more 

 primitive than those of Polyodon §, and as they have not yet 

 been described, so far as I am aware, I propose to do so and 

 to compare their structure with that of the anal and pectoral 

 fins. All three fins — pectoral, ventral, and anal — strongly 

 resemble each other in external appearance, being extended 

 and composed of numerous articulated dermal rays, at the 

 base of which there is in each case a similar muscular lobe 

 projecting beyond the body-wall, and in which the series of 

 baseosts is imbedded. 



On dissection the anal fin is seen to be supported by a 

 series of cartilages, baseosts, 21 in number, which articulate 

 internally with a similar series of axonosts. The latter, 



* Am. Nat. xxi. 1887, p. 1017. 



f Cat. Foss. Fish. (4 vols. 1889-1901) and Vert. Palaeont. (1898). 



\ Presidential Address to Zool. Section of Brit. Assuc. (1900). 



§ St. George Mivart, in lb79 (Tr. Z. S. x. p. 457), described and 

 figured the anal fin of Polyodon as the ventral, the mistake being due to 

 a wrongly labelled specimen in the Museum of the College of Surgeons, 

 but it is curious to note that on receiving Thacber'a paper he did not 

 realize this, but supposed the difference to be due to individual variation. 



