se The Homologies of the Fins of Fishes. 415 
rays, however, have developed as a result of specialization of less 
important and more delicate hair-like bodies, described by pre- 
vious authors, which traverse the fin from base to border within, 
and which Prof. Ryder has called actinotrichia, and which he 
says originate in the mesoderm. 
I have determined that continuity of neural spine, axonost and 
baseost is characteristic of all primitive fishes; while these are 
again to be distinguished into types which preserve the primitive 
actinotrichia, and those in which fin-rays are developed. In the 
latter case the fin-rays are always continuous with the baseosts or 
axonosts. (Fig. 7.) It has been long known that in modern 
fishes the axonosts have severed their connections with the 
neural spines, and do not correspond with them in number or 
position. The axonosts are then termed interneural and inter- 
hæmal bones. It is also a character of modern fishes (most 
Teleostomata) that the basilar bones are rudimental or wanting. 
The gradations between the primitive and modern types of fins 
represent the evolution of the class of fishes, and I have en- 
deavored to express this view in the system which I have pro- 
posed, as above cited. (Am. Nart., October, 1889.) 
The various stages or types of dorsal and anal fin structure 
may be defined as follows; and it seems necessary to give them 
names, to avoid the circumlocution requisite to express the 
characters in any other way. The diagrams appended explain 
the types referred to. They are applicable to the dorsal fin, and, 
inverted, to the anal fin. Each separate series in its totality is 
called by Ryder the actinophore. Its kinds are the following : 
I. Actinotrichia’' present. 
Neural spine, axonost and baseost continuous; £xfotetramerous. 
II. Fin-rays present. 
All the elements present and continuous ; Ectetramerous. 
All the elements present, the axonosts not an- 
ticulated with the neural spines; Ectrimerous. 
Like the last, but the baseosts rudimental or _ 
wanting ; Ecdimerous. 
Axonosts not connected with the neural spines, 
reduced to one or two for each fin ; Rhipidopterygian. 
