III. — Ventral Fins of Ganoids. By James K. Thacher. 



Es erscheint als eine allgemeine Regel, dass homodyname, ungleichartig entwickelte 

 Korpertheile an den hinteren Absclmitten indifferentor erscheinen als an den vorderen, 

 dass besonders die Gliedmaassen des Schultergiirtels viel reichere Umgestaltungen 

 eingehen als jene des Beckengiirtels, an denen sicb das urspriingliche Yerhalten langer 

 und vollstandiger bewabrt. — Gegenbaur, Jen. Zeitschr., Bd. v, p. 417. 



In February last I published a paper* to show that the limbs with 

 their girdles were derived from a series of similar simple parallel 

 rays, and that they were a specialization of the continuous lateral 

 folds or fins evidenced in embryos, which were with some probability 

 homologous with the lateral folds or metapleura of the adult Am- 

 phioxus. The following short paper is designed to do little more 

 than coiToborate the views there stated. 



In PI. LIX, fig. 64 of that previous paper, an Acipenser ventral is 

 exhibited. Tlie ventral of another individual of the same species, 

 Acipenser b^-evirostris, is figured here, PI. I, fig. 1. Comparing these 

 two we find certain differences. In the first place the number of rays 

 is not the same, being seven in fig. 64 and eight in fig. 1. But we 

 see that the orad ray of fig. 64 is very broad, as it is also in fig. 65, 

 which exhibits the other ventral of the same specimen, and in the 

 text we read " the breadth and outline of c (the orad ray) raises a 

 suspicion of its double character." This suspicion, then, is confirmed 

 by the eight rays of fig. 1, where the orad ray is slender like the 

 rest. But there are other differences, and these, aside from insig- 

 nificant difterences m the terminal segmentations, lie in the manner 

 in which the originally separate rays have united with one another. 

 In fig. 64 the aborad three rays are all separate and complete, while 

 in fig. 1 there is quite a different state of things. Here the proximal 

 part of 8 has united with the adjacent part of 7 ; 7 is free from 6. 

 But the proximal segment of the latter, which is free in fig. 64, is 

 here, in fig. 1, joined to 5. But 5 is free from 4 and 4 from 3 in fig. 

 1, as is not the case in figs. 64 and 65. 



The part bd is a large flat piece of cartilage of about the thickness 

 of the rays. The process b bends slightly downward (ventrad) and is 

 a little foreshortened in the figure. It approaches its fellow on the 

 other side, but does not unite with it. 



* Median and Paired Fins. Trans. Conn. Acad., Vol. iii, 1877. 

 Trans. Conn. Acad., Vol. IV. .30 Dec, 1877. 



