416 The American Naturalist. [May, 
As I have already pointed out, there are two types of the 
Rhipidopterygian fin, the Rhipidistious where baseosts are 
present (teste, Traquair), and the Actinistious (Fig. 8) where 
they are wanting. These divisions, corresponding to orders, do 
not require renaming. 
No fishes are known with the actinophore dislocated from the 
neural spines, and retaining actinotrichia. Should such be dis- 
covered, they will appropriately receive names similar to those 
given to the series with fin-rays, with the prefix ento, as ento- 
trimerous, entodimerous, etc. For obvious reasons the discovery 
of an entotrimerous fish is more probable than that of an ento- 
dimerous one. . 
Ac | : | Fr 
an ae 
| 
ee ee 
oe 
Fic. 7—Diagrams of actinophores; A, entotetramerous; B, ectetramerous; C, ec 
trimerous; D and Æ, ecdimerous; Ac, actinotrichlia; Fr, fin-rays; Ba, basiost; 4%, 
axonost; Zc, intercentrum, 
| 
fos red 
1 
As regards the origin of the actinophores, Gegenbaur suggests 
that they were derived by segmentation from neural spines, while 
Thacher believes that they originated independently of the latter. 
Thacher bases his views on the structure found to exist in Selachii, 
which he supposes to represent the most primitive condition. The 
Selachii must, however, be regarded as, in many respects, degen- 
erate, and Xenacanthus is probably more primitive. It is proba- — 

