THE OSTEOLOGY OF AMIURUS CATUS. 307 
anterior ray of the pectoral fin also points to the lapse of a consider- 
able period of time, during which small successive changes have been 
wrought, and the extent of the modifications of the dorsal fin for the 
same purpose point to the same conclusion. Other evidences of a 
similar nature are to be seen in the absence of any neural arches 
corresponding with the hemal processes which support the rays of 
the caudal fin, and in the complete abortion of the radialia of the 
ventral fin. 
All these latter points are, however, subordinate to the first in 
determining the relative position of Amiuwrus. Since the course of 
development, as is shown both by the ontological history of any 
form, and by the study of the various vertebrate groups, leads from 
a purely cartilaginous to a purely osseous skeleton, the amount of 
cartilage present in the skeleton of any fish is in indirect relation to 
the extent of its development. This character is necessarily less 
subject to the modification of external conditions than other parts, 
so that even though certain of these may undergo great specialization, 
yet if a considerable amount of cartilage be present in the skeleton, 
the form under consideration must be considered as standing com- 
paratively low in the group. The Lophobranchiates, for instance, 
have undergone modifications, even more striking than those of 
Amiurus, but since the relative amount of cartilage in the skull is 
greater, and the parts modified may all be readily influenced by the 
conditions of existence, the members of this group must be placed 
lower among the Teleosts than Amiurus. 
In conclusion, a few words concerning the process of ossification. 
From what has already been said in this paper, it will be seen that 
what may be termed several modes of ossification are present. We 
have, in the first place, the deposition of the bone in general connec- 
tive tissue, forming certain of the ‘ Deckknochen,’ and the bones 
around the mucous canals ; we have, secondly, cement-bone, as in the 
premaxille and dentary ; and we have, thirdly, perichondral bone, as 
in the prootic, palatine, etc. It has also been shown that all these 
forms of bone formation pass into one another perfectly, no dividing 
line marking the termination of one form and the commencement of 
another. Not only, however, do they thus pass into one another, 
but they also replace each other. This is very evident in the. case of 
the frontal, maxille, vomer, parasphenoid and mucous canal bones. 
At one time these bones were probably formed by the union of the 
